A seam is where two parts of fabric are stitched together - what Rarity is doing in the picture...

Sweetie Belle said there was something wrong with the sewing machine, then Rarity is saying it 'seams' - as in 'seems' - fine. Basic pun!



Rarity, Sweetiebell, and MLP:FiM is (c) of Hasbro

A lot of you are confused by the joke so here's an explanation (thanks to):