This is a natural sign.
A natural sign temporarily cancels the effect of a sharp or a flat. For example, in Pieces of Eight, there is a flat sign at the start of the line, which tells you to play all the B notes as Bb. However, in bars 1, 2 and 3, there is a natural sign before the last B note in each bar. This natural sign cancels the effect of the flat sign at the start of the line and means that you play these notes as B instead of Bb.