This is a natural sign. A natural sign cancels the effect of a sharp or flat for the rest of that bar, or until another sharp or flat sign occurs within that bar. Notice the alternation between F natural (F@ ) and F# in the following example.
Now try this 12 Bar Blues solo which makes use of sharp, flat and natural signs. Breath marks are only shown every four bars here, but take a quick extra breath every two bars if you need to.
This 12 Bar Blues solo makes extensive use of the note Eb.