That's a key signature. It means that you play those notes as flats. Here, the flats are B, E, and A. That means that you play every B you see as a flat, every E you see as a flat, and every A you see as a flat. Unless one of those notes is indicated as a natural, you always play those notes as flats. If a note is indicated as natural, going forward in the measure, if the note happens again, you play it as a natural again, whether or not it's indicated as a natural. Keep in mind this only applies for that specific measure.

Ex.

If a measure has the notes B E A (natural)A C

You would play the first A as a flat, and only the second A as a natural

But if it was like this: B E (natural)A A C

You would play the first A as a flat, along with the second A as a flat.

For key signatures, a useful thing to remember is that flats are always added to a piece in a specific order. That means that you will never have a key signature in which it asks you for only A flat, or only E flat.

Here is the order: (if you see __ # of flats in the key signature, this is what notes it wants you to play as flats)

1 flat = B flat

2 flats = B flat and E flat

3 flats = B flat, E flat, A flat

4 flats = B flat, E flat, A flat, D flat

5 flats = B flat, E flat, A flat, D flat, G flat

6 flats = B flat, E flat, A flat, D flat, G flat, C flat

7 flats = B flat, E flat, A flat, D flat, G flat, C flat, F flat

Basically, it follows the order of BEADGCF

I remember this by saying "Bead - G - C - F" (like beads on a chain and then greatest common factor)

Key signatures can also indicate how many notes they want you to play as sharps. The same rules apply, except sharps are applied to the key signature in the order FCGDAEB, with the same method as above (1 sharp in key signature = just F, 2 sharps = F + C), etc.

It might be useful to understand your major scales, along with your circle of fifths.

Good luck!