Often slash chords are just inversions of the root chord
1 over 3 means play the 1 chord, with the 3 note of the scale in the base.
The top half of the slash is the chord to be played. The bottom half of the chord is what note the base should be. Eg in C major 1/3 can also be notated as C/E
4/1 chord - C Major scale means you are playing the F major chord but put the C note on the bottom, also known as a 2nd inversion of the F major chord.
Jeffrey uses For the Sake of the World to illustrate.
5/1 – G Major key. 5 chord in key of G major and throw the 1 on the bottom.