probably the creator of the community want to select who can enter and who cannot, it's like for private Instagram profile, the owner have to accept your request
There's no such option on Lemmy at the moment.
It can mean two things:
A) the instance is overloaded, which happens frequently with lemmy.ml
B) the instance has defederated from your home instance
Sorry but that's wrong. The "subscribe pending" thing is just a known bug currently. It means you're actually subscribed but the server on one end hasn't acknowledge it yet due to load.
You can test it out yourself, "subscribe pending" communities show up in your "Subscribed" feed.