I kept stuff like that vague so you could apply it to whatever game system you were playing, but I like to use the “small encounter” for stuff like finding an NPC or finding a small group of goblins or smth. Whatever would count as as a minor event.
For example, in my current game the characters are exploring tunnels under a mine. I rolled small encounter and they encountered a group of rock goblins who wouldn’t let them pass without a toll. They could have fought and probably easily killed them but who knows how many more would come? The characters just decide to pay the toll and move on.
In that same dungeon later, I rolled a regular encounter which the oracle said would be “challenging” so I found myself facing a giant rock monster that killed two hirelings and crippled a PC ;(
As far as inspo for what the encounter could be, I recommend you just use the encounter tables for the game/system you’re playing.