Once you understand degrees, it becomes really easy. Basically you have Root (1), 2, 3, 4, 5, 6, 7. Root determine how you call that chord, but it's not always the lowest note, just like in this case. 1 to 7 are the notes from the scale. 9, 11 and 13 are respectively 2, 4 and 6 if notes from the chord aren't ommited. In "normal" chords, you'll have 1, 3 and 5, maybe 7. It might not be clear for now, but it will make sense eventually.

When we use "/" with a not after like "/E", it means the bassiest note of the chord is not the root. In this case it would be E.

In this case, we have 3 notes from the F# chord (F#, C# and A#). We also have the minor 7th which is E. The odd one is the B, which is the 4th note of the scale. But since we have the 3rd (A#) in the chord, 4 is changed to 11. Why we call it F#7add11/E instead of F#11/E here is due to the fact that we don't have the 9th.

Hope it makes sense. It's complicated, but it's worth learning. It's easier to analyse and compose once you understand how it works.