Two main ways fortnite keeps the game "fresh" is adding new "skins" (player character appearance) and "emotes" (actions). They licensed the adventure time and spongebob characters as "skins" which players can earn or purchase.

The adventure time characters appear to be doing a basic "hug emote", which is one of several interactive emotes that, if one player does it, another player can reciprocate by approaching and engaging the emote. It looks like they're making out because this emote was designed for basic skins, not specific to these skins.

I'm not sure if that's just spongebob's face or not, but I assume it's an emote exclusively designed for that skin (see Carl's comment reply below)

Edit: The primary game mechanic is third person shooter, though they've got several popular roleplay style game modes/options where folks just fuck around. It's become kind of like roblox, in which it's now a platform for game makers to make their own game and publish it for others to play