An edge is any advantage one side holds over the other. In casinos the word almost always means the house edge: the built-in mathematical margin that lets a casino profit over a large number of bets, while individual players can still win in the short term.
The house edge is expressed as the percentage of each stake the casino expects to keep on average. It varies widely by game and by rule: some blackjack and baccarat bets sit around 1% or lower, single-zero roulette sits at 2.70% and double-zero roulette at 5.26%, while many side bets are far higher. It is the mirror image of RTP (return to player): a slot with 96% RTP has a 4% house edge. Players can gain an edge themselves only in rare cases of advantage play, such as card counting.
Edge and your game mix
Many providers release games in several RTP configurations. The version you choose, within what your licence allows, sets your theoretical margin. See casino games integration.