H&M stands for Hennes & Mauritz.
It used to be just called "Hennes" when it started off as a Swedish female clothing shop in the 1940s. The owner of the company joined it up with a men's hunting clothes shop that he owned and it became known as Hennes & Mauritz since then.
But this was simplified to H&M when the company wanted to spread out to other countries. Most people would have struggled to say the full name (I do anyway).
I like their fashion range. Affordable and trendy is the vibe they are going for, and it's nice to be able to find clothing that doesn't have lots of branding on it. That's why I shop at H&M and Uniqlo.