Triads are three-note chords. Inversions refer to the various ways you can arrange those three notes, and “first inversion” refers to the standard arrangement, which starts with the root note of the triad, goes to the third, and ends on the fifth. For instance, the notes in a C+ triad is C-E-G.
If you haven’t gone through the lessons prior to this one, you may find yourself a little stumped… if that is the case, please go back and review the others leading up to this – all of them! :).