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Hello @DHeaton ;
Hopefully, the Capo Comprehension course that @Kitman suggested will answer your question. If you have any more questions, please don’t hesitate to ask again.
The simple answer to your original question, is the capo changes the key. It’s like moving the nut up the neck. If you play an E major chord, the 6th string is your lowest E note and is called you “root” note. So as you move that capo up the neck, that note changes. But the relative position of the notes stays the same, so you are still playing a major chord.
The note on the 6th string and 5th fret is an A. So if you put the capo on the 5th fret, your open 6th string will be an A note. If you play an E chord, or more accurately, an E “shaped” chord, you will be playing an A major chord.
This is called a moveable chord. This is possible because the relationship between the strings remains the same. When you move from the 6th string to the 5th string, you are moving up in pitch the same amount as if you move up on one string 5 frets. So the 5th fret on the 6th string is an A, and matches the open 5th string.
I hope this helps.
MG 😀