When you start learning the guitar, it’s not long before you come across the musical alphabet: A B C D E F G. Having played a few tunes using these notes, you’ll soon notice that some frets are missed out. Take the high-E string, for example. Fret 1 is F, then you skip the next fret on your way to G on fret 3.
What about those in-between notes?
Back in the mists of Western musical history, when notes were named, only the 7 notes A to G were used. Actually, that’s not entirely true, there was another note, H, but we don’t use that name anymore. The in-between notes weren’t used, so they didn’t get named.
Sometime around the 17th and 18th century, the twelve notes of the octave that we use today were set, but instead of give some handy letter names to the extra 5 notes, instead two symbols were added.
The hashtag-looking symbol is a sharp sign and the odd-b-shaped symbol is a flat sign.
The sharp sign indicates the note one fret higher. The flat sign indicates the note one fret higher.
All the in-between notes can be described using either sign. For example, that second fret note on the high-E string could be called F-sharp (F#) or G-flat (Gb).
The link below is to a free single page PDF explanation of this for the guitar. Enjoy!