Mothers of Reading - ancient vowels for the consonant only text
Nikkud was developed around 9th century AD. Even during Old Testament times, an older system to help figure out the vowels was invented.
י - (Yod) was inserted in the word after a consonant to indicate long vowel I or E.
ו - (Waw/Vav) was used for long O or U
ה- (He) generally used at the end of a word to indicate mostly A or E
א- (Alef) used mostly for long A
So you could find words written with or without these helper consonants which stood for vowels instead. E.g. יהושוע vs יהושע (Yehoshua), notice the missing ו at the end. For more see:
- https://en.wikipedia.org/wiki/Plene_scriptum
- https://en.wikipedia.org/wiki/Mater_lectionis
The system of "mothers of reading" is employed all the time in Modern Hebrew, with extra rules and more consistency, as opposed to the ancient more relaxed and inconsistent way.