Hi angels3boys.
allthumbs has it right but it might make it easier for you to understand to just say that the first letter is the chord, the letter after the slash is the bass note to use. The 'default' form of any chord uses the 'root' (1) as bass note of the chord, the note that the chord is named after. So, for example, when you see a chord written as G, you play a G note as the lowest note of that chord.
If you see G/B, you play a G chord, but you make the B note the bass note, the lowest note. The note after the slash is usually either the 1-3-5 of the chord, but not necessarily. You can have a G/F#. F# is the 7 of G, so in this case you have a Major7 chord which uses the 7 as bass note.
Because of the slash used in writing these down, they are all known as 'slash chords'.
Until you become aware of where these bass notes are on the fretboard, you can simply play the chord as indicated by the first letter. Slash chords are just a more detailed look at how to play the chord. In other words, a G/B is just a G chord.
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