Yest stands sixth, writes the vowel e, and carries the numeral 5 from Greek epsilon. Its name is the third person singular of the Slavonic verb to be, the word for it is, which places one of the load bearing words of theological language in the sixth position of the alphabet.
Old Church Slavonic distinguished this plain e from the sequence of a glide plus e, and wrote the second with a separate iotified letter. The distinction matters for reading the earliest manuscripts and disappeared from most later traditions, which is why yest looks in a modern alphabet like a single unremarkable vowel and in an early one like half of a pair.
As a numeral it is 5, and it therefore does heavy work in a liturgical book: the fifth tone, the fifth week, the fifth ode. In the running heads and marginal numbers of a Gospel book it is one of the letters most often seen under a titlo.
The letter survives in every modern Cyrillic alphabet, and in Russian it acquired a companion in the eighteenth century when a letter for a different vowel was formed by adding two dots above it. That companion is still treated as optional in ordinary Russian printing, which makes it one of the very few letters in any alphabet whose use is discretionary.