Yestu writes the vowel e and carries the numeral six. Its name is the third person singular of the verb to be, meaning it is, and it completes the second clause of the recited alphabet: glagoli dobro yestu, to speak is a good thing.
The vowel is a front mid vowel, and Old Church Slavonic distinguishes it from the separate letter yat, which writes a different front vowel whose exact quality varied by region. That distinction is one of the working tools of Slavic philology, because the two letters were confused in different ways in different places, and the pattern of confusion in a manuscript indicates where the scribe learned to write. A text that keeps them apart consistently is following a stricter norm than one that does not.
The letter also stands at the front of a large group of words in which a glide precedes the vowel. Glagolitic has a separate sign further down the order for the sequence with the glide, so the two are distinguished in careful writing, and the way a given manuscript handles the pair is another of the small habits by which its tradition can be identified.
The numeral six comes from counting the Glagolitic letters in their own sequence, which by this point in the order has already moved a full step away from the values Cyrillic assigns to the same names.