Literally, 'four-lettered word'. Pronunciation, thanks both to the unwillingness to pronounce such a holy name and the standard
Semitic complete lack of weak vowels in the writing system, is up for grabs. Modern interpretations are Yahweh and Jehovah, both probably incorrect.
Also sometimes seen as YHVH, IHWH, JHVJ, IHVH, and a whole slew of other linguistically accurate strings of phonemes.
Either way, it was considered magically powerful during the middle ages.
Instead of re-writing this, I'll just append a little note. The whole problem here is generally called ketib-kere, or "what is written" and "what is read". In other words, the consonants of God's name are clear, and were already clear when the Hebrew Bible was standardised: Yod-He-Waw-He, or YHWH. Variants of the letters can be attributed to different transcriptions and pronunciations.
Now, when the good Masorites were busy adding vocalisations to the Hebrew text, at a time when Hebrew was dead or dying as a living language and there was a need to preserve the pronunciation, they decided, for whatever reason, that the proper name of God should not be available to those not part of the tradition. There are older manuscripts which write the name in older, aramaic script, while the main-text is in the familiar quadratic script. When the Ben Asher Masora was more or less finalized, they took the consonants and added the vocalisation of the word adonai, "My Lord", giving us Yahweh or Yehowah.
The thing is, however it is written, the tetragrammaton is still pronounced adonai when the text is written YHWH. Hopeless? Well, we do have clues as to how the name was originally pronounced, mostly from The Name as a theophoric element in Hebrew personal names from non-Biblical sources. Since these are rather rare, and even then unvocalised, The Name is still the product of guesswork.