Sharp (#) and flat (b) are two musical terms which essentially mean: ‘higher’ and ‘lower’.
Here are all of the notes in order:
A A# B C C# D D# E F F# G G# A
A# is one semi tone higher than A,
G# is one semi tone higher than G,
E is one semi tone higher than D#,
Bb is one semi tone lower than B,
and Db is one semi tone lower than D.
This means that A# is the same note as Bb (since there is only one note in between A and B). Don’t worry too much about this now, as it will become clear why these terms are necessary later on.