Accidentals are Music symbols that change the Pitch of a Note and include Sharps, Flats, Natural Signs, Double Sharps and Double Flats.   Each type of Accidental can either raise or lower a note by a specific distance or amount known in music as an Interval.
Sharp     raises a note by 1 semitone.

Flat         lowers a note by 1 semitone.

Natural     cancels an accidental.

Double Sharp     raises a note by 2 semitones or 1 Whole Tone.

Double Flat     lowers a note by 2 semitones or 1 Whole Tone.

In Music the smallest distance between any 2 Notes is known as a Semitone and can be seen on a keyboard as one key to the next or on a guitar as one fret to the next.   Two semitones are called a Whole Tone and usually occur when a Double Sharp or Double Flat are used.

Guitar Fretboard - Sharps

Guitar Fretboard - Flats
When you look at the Piano black keys, they don't have their own names so they have to borrow from the white keys nearest to them.   The same applies to some of the Frets on a Guitar Fretboard.

Notice that the piano doesn't always have a black key between 2 white keys the same as some frets on the Guitar?   Specifically the notes B-C and E-F on the Piano don't have a Black key between them.   The same principle applies to the Guitar and all other instruments.

Earlier you saw on the Piano that the black key notes can have 2 names, for example C# and Db are the exact same piano key.   The same concept applies to B, C, E and F since  B & Cb  or  B# & C  are the same note on instruments.   This is called an Enharmonic Equivalent.   You can also see that when we add Double Flats or Double Sharps any Note, Piano Key or Guitar Fret can have several names.   Since the same principle applies to the Guitar, this further shows you that Music Theory is universal and can be applied to All Instruments.

Enharmonic Equivalents

When a semitone has 2 notes with the Same letter name, it is called a Chromatic Semitone.

When a semitone has 2 notes with Different letter names, it is called a Diatonic Semitone.   It is easy to remember since Diatonic and Different both start with the letter D.