You can use the internet for a few things including some games, without PS Plus. It does give you a few games a month to keep, and lets you play online with any game that supports it.
Without it you can still play ‘free to play’ games online like Rocket League or Genshin Impact. These games normally have in-game purchases etc. that are optional, but don’t require PS Plus to play. You can also use video apps, buy games from the Playstation Store, send messages to friends, etc. without Plus.
Note that on Xbox it is different and you need to have Xbox Live Gold to use even video apps or Free-to-Play games.