if OP is looking for a simple answer, without delving into linguistic theory, look at it this way:
The rules of Hebrew grammar are rigid. Between every 2 consonants, you must insert a vowel. The vowel which you insert is determined by permanent rules, so that you don’t even need to write the vowel down–any native speaker of the language knows which vowel to pronounce between the 2 consonants.
Simple examples:
in one class of verbs, every present-tense verb is written with 3 letters (consonants), but always has two vowels inserted between them to make a 5 letter word.
KTV is always pronounced k-O-t-A-v – “He writes”
NTN is always pronounced n-O-t-A-n – “He gives”
to make these verbs past tense, you just change the vowels, to an A and another A
KTV is always pronounced k-A-t-A-v — “He wrote”
NTN is always pronounced n-A-t-A-n —“He gave”
The rules are easy to learn, very rigid (almost no exceptions to confuse you) and once you know them, you start to wonder why anybody ever invented vowels.
One problem is that you have to decide whether the sentence is written in present tense or past tense before you can read it. But once you know that (from the context of ther rest of the sentence), it is impossible to mis-pronounce the word.
It’s a little like these sentences in English: “I read the newspaper yesterday”
“I read the newspaper every day”
You don’t know whether to pronounce “Read” as “reed” or as “red” until you see the next 3 words. That happens a lot in Hebrew.
But once you know the context, the pronunciation of the vowels is obvious, and they don’t need to be written down. (this was probably very convenient back in an age when writing involved using a hammer and chisle on stone.) 