Hola Nicole

It's easier to understand it with these examples:

1. Yo olvidé las llaves. I forgot the keys.

2. Se me olvidaron las llaves. I forgot the keys.

These sentences are saying basically the same. The first one is more straight forward, so you have "yo" as the subject and then the verb "olvidé" conjugated in the yo form. However the second one is  more of a complex structure called "se accidental", where the verb needs to agree with what you forgot (las llaves), not the person who forgets. Plus, you use the reflexive "se" and an indirect object pronoun (me) indicating who forgot. 

I hope this clarified it.

Saludos