A tactic in chess is a sequence of moves which threatens to win a piece or force checkmate. Common examples young players need to learn are forks, pins and skewers. Each of these will be explained by clicking on the menu to the left.
Like all skills (football, music, maths, . . .) practice is needed. The Lichess practice page has a section on basic tactics with several examples of each type for you to try to spot the tactic with a great move.