To make a long story short, initially Hebrew (and any other Semitic) alphabet contained only consonants. But at some point (somewhere 3000 years ago) writing "just consonants" became not sufficient. Gradually, the ancient Hebrews and Phoenicians started to use some of the consonant letters for the indication of long vowels. It started probably with Hei (ה) at the end of a word, to mark any vowel, then the letters Vav (ו) and Yud (י) became involved, and later Aleph (א) joined on. With the time a whole system of rules was developed in Hebrew, which vowels should be marked with letters, either with vocalization (the Nikkud) or without it. In Yiddish, the letter Ayin (ע) is used to indicate the presence of the sound E.

Those letters (basically consonant letters used as vowels) became known under Latin name of Matres Lectionis (Mothers of Reading.)

Disclamer

Some people believe that saying "such and such letter reads as such and such vowel" is not quite correct in the case of Matres Lectionis, but it would be much more accurate to say "such and such letter indicates the presence of certain vowel".

I'd say, first, it's arguable, and second, it doesn't really matter for practical purposes of learning Hebrew.