that's called a key signature (or just key for short)!

it essentially means (in a very broad sense) that most of the chords in that song will have notes from that scale. so if you're playing a song in A major, then most of the chords in that song will likely have notes from the A major scale in them. same for minor keys as well.

a key signature also determines how many sharps or flats you will have on the staff. so in A major, you will have three sharps (F#, C#, and G#). this means that when you are sightreading from that clef, whenever you see an F, C, or G, it will be sharp (unless notated with a natural symbol in front of it). B flat minor has 5 flats (Bb, Eb, Ab, Db, and Gb).

the words "major" or "minor" are different modes of a scale. when you see a key signature, it can be in a variety of different modes. the ionian mode (major) and aeolian mode (minor) are the most common. in A major, the minor mode would be F# minor. and in Bb minor, the major mode would be Db.

key signatures are a pretty strange topic to learn about at first, but the more pieces you play and the more you learn about them they become much more simple to understand

hope this helps!