It's a broad category.

Good health practices are the foundation--diet, exercise, hygiene, sleep, hydration, etc. Then it comes down to things that contribute to your happiness. Things that just kill time aren't really fulfilling for the most part, you want something that feels like you're actually developing some happiness in a more time-investment sort of way. This usually involves either creating or improving something in some fashion. What form this takes varies tremendously from person to person, for some its learning, it can be art, it can be gardening, it can be social activities, it can even be watching movies if you're actively consuming, where you're thinking and growing as you watch. Common with movie buffs who can break down the ins and outs of films, they acquired that skill by watching and thinking about lots of different movies. For a big sports fan, watching or playing sports can qualify.

The key that binds them all together is there's something that can progress, something that you value that can be built further up or otherwise gained.

When they say self care, they want you to think about things in this way. Like, smoking a whole shitload of weed might be really enjoyable, but is there anything developed or gained there? If not, even though it might feel good, that wouldn't be self care.