H - Meaning in Japanese

In Japanese, the letter H stands for the word ecchi エッチ, and it means something along the lines of "sex," "sexy," "sexual," or even a bit "sexually perverted."

Manga: "He is My Master," Kore ga Watashi no Goshujin-sama これが私の御主人様 (Chapter 1)

• Context: a girl doesn't like her uniform.
• nani itteru no yo
  なに言ってるのよ
  What are [you] saying?!
• konna ecchi na fuku wo
  kiseru tsumori!?
  こんなＨな服着せるつもり！？
  [You] intend to make [us] wear such perverted clothing!?
• senaka wa konna ni deteru shi!
  背中はこんなに出てるし！
  The back is getting out this much!
• It's this much exposed!
• sukaato wa chou mijikai shi!!
  スカートは超短いし！！
  The skirt is super short!!

So the name of the manga and anime B-gata H-kei B型H系, for example, would mean "[blood] type B, perverted (ecchi) type [of character]." (in this case, B-gata refers to a superstition about personality, where people with type B blood are creative and cheerful, but irresponsible and impatient)

As for why H means ecchi, it's because the name of the letter H in English is "aitch." And when "aitch" is katakanized, it becomes ecchi.

It's originally the reverse: the word ecchi is the one that comes from the letter H. And it comes from the letter H because it's the first letter in the romaji of the word hentai 変態, which's used to call someone a "pervert," among other things.