Generally speaking, it's a business model where there is significant post-launch content as a means of getting a player to keep returning to the game. Moreso then just one DLC drop at some point, on a more recurring basis then that.
Sometimes it's free, like Nintendo has done with Splatoon 2 and ARMS. Like Splatoon's Splatfests and occasional new levels are quintessential GaaS stuff. It's a reason to keep coming back to the game you probably would've moved on from otherwise.
But that's also the minority.
Publishers generally use the GaaS model to get people to continue interacting with a game in order to sell DLC or Microtransations. Sometimes the model itself is just DLC, where the new content drops are just behind a paywall. Like well, basically every Destiny expansion. That's what people criticize it for, is when it's just used as a scheme to bilk more money out of you.
Sometimes it's free, like Nintendo has done with Splatoon 2 and ARMS. Like Splatoon's Splatfests and occasional new levels are quintessential GaaS stuff. It's a reason to keep coming back to the game you probably would've moved on from otherwise.
But that's also the minority.
Publishers generally use the GaaS model to get people to continue interacting with a game in order to sell DLC or Microtransations. Sometimes the model itself is just DLC, where the new content drops are just behind a paywall. Like well, basically every Destiny expansion. That's what people criticize it for, is when it's just used as a scheme to bilk more money out of you.