In American English, the letter H is always pronounced "aitch". A growing trend in the UK, however, is to pronounce it "haitch". Linguists agree that the former is the standard pronunciation, but in British English the latter is considered an acceptable variant.
Lars picked up the nonstandard pronunciation from, in his own words, "watching too many Pyrocynical videos". As such, he will pronounce H as "haitch" on the podcast. While this does not come up extremely often, the semi-frequent mention of h3h3Productions brings it out.
This is not unusual for him, as he enjoys many subtle linguistic variants, such as "an historic".
In later episodes of the show, Joe started feigning indignation whenever Lars does this, leading to some form of the following exchange:
Joe: It's aitch, Lars. Aitch.
Lars: Yes, that's what I'm saying. Haitch.