This means to unexpectedly meet someone you know or know about. For example:

I ran into a few former coworkers there. That was a nice surprise.

You'll never believe who I ran into at the supermarket: Leonardo DiCaprio!

A common place to "run into" someone is "on the street":

I can't believe I just ran into you on the street like that. What a coincidence!

That means outside in a public place, like on a sidewalk.