I bet you remember 集める means "to collect something." But this time you're not collecting something. Instead, something just… collects. In other words, 集まる means to come together or to gather. Friends come together, or fans gather at a venue — that sort of thing. No direct object needed, since this is intransitive.

Verbs ending in okurigana that rhyme with ある tend to be intransitive like this. You can remember this because if something is ある, it's just there, existing, not acting on anything else. That's why verbs that sound like it are often intransitive too!

The reading is the same as 集める so if you know that word you'll know how to read this word as well.