mater lectionis — a consonant in Hebrew and some other Semitic languages that is used to indicate a vowel (niqqud — the diacritical marks that appear in some Hebrew text above and below the letters — isn’t used consistently and wasn’t an original part of the written language). In some cases the words can be spelled either with or without the matres lectionis.
in Hebrew these are:
א (which has basically lost its original consonant sound)
ה
ו
י
The tendency of Hebrew (et al) to leave out the vowels — the clues to pronunciation — reminds me in a roundabout way of tonal languages. Someone once told me that in some cases people who speak extremely tonal languages can communicate at a distance with just the tone (whistling/drumming), without the actual word. To achieve communication this way, you obviously have to have a fairly good understanding of the language. Likewise, you have to have a decent background in Hebrew to be able to guess at the correct vowels every time (in my not decently backgrounded opinion). It’s pretty neat — like you’re communicating in code. (But I guess all language is really just code.)
This was always the part that made Hebrew (and Arabic) difficult for me. When you ask a native speaker how they know what the correct sound is you will all get the same answer, “You just know.”