CAN ANYONE EXPLAIN ME SOMETHING, PLEASE?

Hello guys,
In this league i playin first time ice shot build.
My question is:
How "You can apply an additional curse" works? I should have this implict on my Yoke of Suffering amulet.
Which additional curse?
I check this on POB and its huge increased my damage but dont know how it works.
Maybe it works in something way with Curse Enemies with Elemental "Weakness on hit" implict on my gloves?
Can someone explain me this things?

Yes... let's say you have two sources of "curse on hit" (e.g. Temporal Chains and Elemental Weakness). With this "you can apply an additional curse" mod on your gear, you can now apply an extra curse to an enemy, meaning they would be cursed with both EW and TC on hit. Really powerful mod to have for builds that benefit from it.

Without it, you can literally only have one curse or the other at the same time.

by default you can apply only one curse on an enemy, such as frostbite, elemental weakness, temporal chains, assassin's mark. if you have multiple curses in your build but have a limit of one, only one of them will be applied at a time, overriding others.

per each "you can apply an additional curse" mod you have on your gear or passive tree, this limit increases by one. this mod only changes the limit and doesn't inflict any curse on its own. you still need to be able to apply each curse with an item or a skill gem. this mod only changes the limit.

you can use a glove implicit to inflict elemental weakness on an enemy, but if your curse limit is 1, you won't be able to use another curse, regardless of its source. if you curse an enemy with sniper's mark, and then hit with your ice shot, snipers mark will be removed because your hits inflict elemental weakness and your curse limit is 1.

if your curse limit is 2, you can use sniper's mark, then hit the enemy, and both curses will be active on the enemy, giving you more damage.

if your glove has 2 curse implicits, e.g. elemental weakness on hit and temporal chains on hit, then you would need a curse limit of 3 to be able to use sniper's mark with them.

this is most likely why your pob says that "you can apply an additional curse" mod gives you a lot of damage, because you already have a curse somewhere in your setup (maybe mark on hit with sniper's mark?), along with the elemental weakness implicit. normally you can't apply both, because marks are also curses, but with the mod you can.