The writing is confusing, but they are saying where they are indexing idioms in a book. (or something like that) Basically, they are saying "idioms that contain the word **same** are located under the noun in that idiom." Don't feel bad, this is confusing for me also.

This is so badly explained that only Tom got it 100%. If even native speakers got it wrong, no wonder you couldn’t figure it out!

It means everybody is in the same situation. for example.. queuing in a big line waiting for the shop to open. Someone gets annoyed and shouts "COME ONN.. HURRY UP!!!".. you could say to that person "calm down mate, we're all in the same boat here". Meaning, everybody has to wait, everybody is being patient. I also agree with Ian, it's telling you where to find idioms in the book.

As far as I understand they wanted to say that the word "same" stands before nouns and verbs in the idioms which contain this word (same).

to be in the same boat = to have similar issues or problems as the next person.