A buff is anything that temporarily modifies your champion's stats or abilities in a positive way - increase attack, increase speed, and so on. A debuff is something that temporarily harms your champion's stats or abilities - decrease defense, true fear, freeze,... 

Mostly buffs and debuffs are placed by your champions' or your enemy's skills, but in some cases they also come from artifacts, yes, e.g. a shield set on one of your champions will give your whole team a temporary shield buff. 

Buffs and debuffs are a very common in all fantasy/mmo rpg kind of games.

A buff is a positive change on your stats/champions (increase attack, defense, block debuffs...), a debuff is the opposite (decrease defense, speed, poison...). In Raid, buffs are those little blue things on top of your champion's health. Debuffs are the red ones you place on the enemies or the enemies place on you. You can see what champions place which ones by reading their skills. You'll need more ACC than RES has the enemy. 

There's also artifacts that can place debuffs like poison, freeze, taunt, etc...

Hope it helped!

yes thank you very much for taking the time to explain them..Now i finally understand what they are talking about when I watch a YouTube video on a game.  Thanks again.