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What are 'Matres Lectionis'?
To make a long story short, initially the Hebrew (and any other Semitic) alphabet contained only consonants. But at some point (approximately 3,000 years ago), writing "just consonants" became insufficient. Gradually, the ancient Hebrews and Phoenicians began 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 in. Over time, a complete system of rules was developed in Hebrew for 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 the Latin name of Matres Lectionis (Mothers of Reading).
Disclaimer
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 a certain vowel".
I'd say, first, that it's arguable, and second, it doesn't really matter for the practical purposes of learning Hebrew.
However, one must not forget, the letters ו, ה, א and י (used as Matres Lectionis) do not have one specific pronunciation indeed, but they rather indicate one of:
א - can be A, O, E, and even i/ee (ראשון).
ה - can be A, E, and sometimes - O (איפה).
Etc.