It prevents debuffs being applied to a champion. The higher the resistance of the target is compared to the accuracy of the attacker, the greater the chances of the debuff being resisted.

ok thanks for your answer because i am stuck with mountain king, banner. It is recommended to place a resistance banner. link . But I have my doubts about that between resistance and defense 

now i have 165 resistance debuf on him. Is that enough?

I would generally recommend the banner slot for an acc banner. Mountain King, though, doesn't need acc. So for him I would probably go with a DEF banner, though it depends on what you plan on using him for. RES is usually only necessary for arena, since for other places you can usually just get a block debuff champ instead.

u want resist on mk, he is an arena champ.  Resist is extremely important in arena and high levels of doom.  I maxed out all my RES in GH even before ACC

chest, or banner? in resist? Is resist better than def?

banner for sure. chestplates are better used for atk, def, or hp since % based equpiment offers a better stat increase than flat rate. because accessories are all flat rate increases, it makes more sense to do resist or accuracy there since they get the same increase on both.

As to resist being better than defense, if you mean generally, then no, you can't really say one is "better" than the other; it depends on the champion and the areas you plan on using them.

If you mean for mountain king specifically, then yes, absolutely. defense is probably mk's least important stat (other than accuracy, since he doesn't use that).

This is my mountain king. 

i

glove : HP% 50

Chest: HP% 50

Boots: speed

Ring: HP 1800

Amulet: CD 33%

Banner: Defense 215

i am thinking of removing the gloves and critrate instead

it is necessary to remove the banner and replace it with resist?

Ideally, you'd get crit damage gloves with crit rate as substat - but that may make it very difficult or impossible to get enough crit rate on all other pieces. So maybe best crit rate gloves with crit damage substat, indeed. 

Also, don't focus so much on set bonuses, they're not nearly as important as the right main and sub stats. Unless you want to put him in a shield set, as I have done myself, but even then you'd need good shield set pieces with the right stats. 

you need to focus on dungeon progress so u dont waste silver on common/uncommon gear

how do you mean ?

it is important that you still have enough critdamage and critrate or not?

that can only be adjusted by the gear 

Common/Uncommon gear is CRAP.  If you are farming gear in campaign, STOP.  If you can't do level 13 plus dungeons you aren't leveling enough heroes to 60 and spreading your good gear on crappy FW champs.  If u have 2xp running farm, if not use energy in dungeons.  You may only get 1 or 2 upgrade worthy artifacts a week... be patient.  A 4* Epic Weapon at +12 is probably better than the +16 6* atk weapon on that MK.  

I passed level 13 with the dungeons.

but unfortunately I don't get the right gear in dungeons, nor at campane (except fire knight, I'm at level 12 there )

if i depend on dungeons, and need speed boots in hp artifact set. Then I can wait until I weigh an ounce

so I have to be in campaine in Kaerok Castle level 6, there I have a better chance of hp artifact speed boots 

then I can immediately uplift the champions in campain

ugh....the gear you are using is crap.  I don't even need to see the individual peices to know that.

Sets are not that important, the stats are.  Maybe for speed boots, getting those in campaign is ok... But that is it.

is this crap? 

i

yes

what is the mess about that then? Usually what I bring in from dungeons is not even the gear I want. And the stats from dungeon gear are no different from campaign. 

you see

i

With dragon, i'm in stage 16

The stats from dungeon gear are no different? Those common gauntlets have 4 single substats, all single rolls per definition. This epic helmet has three extra rolls on its first three substats - so a total of 7 rolls instead of 4. That's a pretty big difference - though of course you still need to get those extra rolls on the right stats, some epic pieces can certainly be bad. 

But the main issue probably is that you're focused too much on sets, as both Trips and I already mentioned. Sure, gear dropping from dungeons will be in random sets - but that doesn't matter if the stats on it are good enough. A HP set bonus is only 15 percent - you can get as much from an epic piece with a double roll on a HP percentage substat, in addition to all the other stats you'd get from that piece. 

But with some gear you must have a set, otherwise the function does not work, like lifesteal, there must be 4 pieces, you want to get 30% healing.