This is your own dictionary, and nobody else has the same one. It is built from the clips you actually listened to.
Some words land here by themselves: while a clip plays, the system picks out the ones it reckons are worth a second look and drops them in as the speech reaches them. The rest you choose — tap any word you don’t know and it comes here too, with the line it was said in.
Mark a word known and it stops being highlighted in the text and stops being counted as yours.