A license says what rules people have to follow to use something (a logo, in this case). So, it gives an owner control over how the thing is used. With a cc-by-sa license, the owners are basically saying “use it however you want, just credit the author and use a similarly unrestrictive license on what you make with it”. Other things, especially company logos, have a much more restrictive licenses that are more like “don’t ever use this, unless we explicitly tell you to (for an ad or something).”

Trademark registers something and says that other people can’t make a similar thing and license it differently in their own. Like, if you made something kind of similar to the Mickey mouse logo, but changed the size or angle of the ears. A license can’t stop you from using that to sell your products, because technically you’re making your own new thing. But a trademark can stop you, because the new thing is too much like the old one. So trademarks have to be registered, to record who came up with it first, but licenses don’t.

Edit to add: Oh, there are also “unregistered trademarks”, basically common law trademark for something like a logo that already represents something. So I guess the Lemmy logo is an unregistered trademark by now.