If we have a triad, let’s say A-C-E, so 1-3-5, its first inversion will be 3-5-1, so C-E-A. That’s the first, the third and the fifth degrees of the scale, in a different order. Basically, what we do is displace the note with the lowest frequency higher to the register and use the note with the slightly higher frequency as the lowest note of the bunch. See chord, root position and second inversion for more.