The laryngeal consonants were guttural sounds produced in the back of the throat, like English h, German ch, Arabic `ayn or qaf, etc. We don't know exactly what the pronunciations were, but in other existing languages, laryngeal sounds like this tend to have an effect on the surrounding vowels. This happens because your tongue has to movie continuously between the sounds it produces, and so there's some blurring that happens between adjacent sounds.
In particular, laryngeal sounds tend to make vowels near them become more back, low, and round. So for example before a uvular q sound in Arabic, the sound /a/ which is usually something like English a in cat will become back and low, something more like au in caught. You can get a process like this, playing out over hundreds of years:
1. You start with a form like Hewis, where H is some laryngeal sound.
2. The laryngeal sound makes the following vowel low and back, yielding Hawis.
3. The laryngeal sound drops, leaving just awis.
In fact this is how we get the Indo-European root for "sheep" (reflected in English ewe, Latin ovis, etc.). The Hittite form is hawis.
In particular, laryngeal sounds tend to make vowels near them become more back, low, and round. So for example before a uvular q sound in Arabic, the sound /a/ which is usually something like English a in cat will become back and low, something more like au in caught. You can get a process like this, playing out over hundreds of years:
1. You start with a form like Hewis, where H is some laryngeal sound.
2. The laryngeal sound makes the following vowel low and back, yielding Hawis.
3. The laryngeal sound drops, leaving just awis.
In fact this is how we get the Indo-European root for "sheep" (reflected in English ewe, Latin ovis, etc.). The Hittite form is hawis.