E# (E sharp) is a musical note commonly called F. It is called E# when it appears in some scales and chords, such as A#.

The reason why it's called E# instead of F in some scales is that musicians like all scales to have one note with each name from A to G. Therefore the A# major scale looks like this:

A# B# C## D# E# F## G## A#

As you can see, every note is sharp, and three notes are double sharp. This makes people unhappy, so it is very rare to find a piece of music written in A#, instead they are written in Bb.