A capo is used as a tool or a crutch. If you have the music for a song you want to sing, but can't sing that low, you can use the capo as a tool to raise the pitch of the song. You can use it as a crutch when you don't know or can't play the chords listed. It moves the nut of the guitar (fret zero.) When you play a D chord in first position, you play frets 2 3 and 2 on the first three strings.
Putting a capo on lets you use the same relative finger position to play other chords. Put the capo on the 3rd fret and play the D chord shape (2, 3 and 2 frets above the capo) or frets 5,6, and 5 in reality and you play an F chord.
In the following example, the Bb chord is a barre chord and is difficult for new players. Put a capo on the 3rd fret and play a G shape and there it is.

Hey Jude simple version
(Lennon/McCartney)
Beatles
F C
Hey Jude dont make it bad
C7 F
take a sad song and make it better
Bb F
Remember to let her into your heart
C F
and then you can start to make it better

IN D WITH CAPO ON 3
D A
Hey Jude dont make it bad
A7 D
take a sad song and make it better
G D
Remember to let her into your heart
A D
and then you can start to make it better

(chords at the beginning and end of line, formatting did not stay when importing-use your musicality to play it correctly)