Hi Paul ... welcome to GfB&B.
Those are called 'slash' chords, for obvious reasons.
The first letter is the chord, the second is the bass note to play under the chord.
A/B would not be one, really, but A/C# would. Or A/E. Usually, the 'non-root- bass note is one of the other two chord tones (the 3 or 5) but you can also put the 7 (A/G#) or flat 7 (A/G) under the chord too. What happens there is that the bass note actually turns the chord into a new chord: AMaj7 and A7 respectively, in this case.
Nothing to it!
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