Izhe writes the vowel i and takes the numeral ten, the first of the tens in the Glagolitic count. It is the first of two signs in the alphabet for that vowel, and the existence of the pair is one of the features of the Slavonic alphabets that has never been given a fully satisfying explanation in terms of sound, since Old Church Slavonic appears to have had one i and not two.
The usual account is that the doubling follows Greek, where two letters had come to be read as i by the ninth century, and that the alphabet took over the pair along with the order it was built on. If so, the second sign is a piece of Greek inheritance carried into an alphabet that did not need it, which would be a useful piece of evidence about how the arrangement was made. Manuscripts distribute the two signs by scribal convention rather than by sound, and different traditions settled on different conventions, so the choice between them in a given text says more about where it was written than about how it was read.
The name izhe is a Slavonic relative pronoun, who or which. As a numeral the letter is the figure ten, and its position at the head of the tens is a consequence of the Glagolitic system counting its own letters straight through from the first.