H/T, HT, or h/t is short for hat tip, which is itself short for a tip of the hat -- a rather archaic indication of respect or recognition. The abbreviated form is used almost exclusively in social media, and is used to credit a creator of content, someone who inspired the content, or most especially, someone who brought content to your attention.
It appears to have originated in personal blogs, but was popularized on Twitter, where the character limit makes abbreviations especially useful. It is now fairly common across the internets.