To Spread Out
To Extend, To Stretch
intransitive verb, godan verb
Remember 広い? Well this is like that, except a verb. When something does the action of getting wide, it means to spread out, to extend, or to stretch.
You already learned 広げる, which is used when YOU spread something. 広がる, on the other hand, means that something spreads out all on its own. It's worth mentioning that lots of intransitive verbs end in がる, and this is no exception. Just think how about a girl (がる) you know can do everything all by herself. She never needs help from anyone else.
The reading is the same as 広い, too, so as long as you knew that you knew this reading as well.
Can you spread both of your legs more?
The living room spreads from this entrance.
The virus quickly spread to all Japan.