This letter stands thirty fourth and writes the sequence of a glide followed by the vowel a. It carries no numeral value.
Its construction is visible in its shape: it is a ligature of the sign for the glide and the letter azu, written joined. That makes it one of the several compound signs in the alphabet whose parts a reader can still see, and the alphabet lists disagree about whether such compounds should be counted as letters at all, which is one reason the total given for Early Cyrillic varies between authorities.
Its history is unusual among the iotified letters, because the sequence it writes never went out of use. Every Slavic language still has it, and every Cyrillic alphabet still needs a way of writing it. What changed is which sign does the job.
In the East Slavic area this letter fell together with the little yus, whose nasal vowel had been lost, and the two collapsed into one. The shape that survived was the little yus and the function that survived was this letter's. The modern Russian letter for ya is therefore a case of a sign and its meaning coming from different ancestors, which is set out in the companion article. This letter itself did not survive into any modern alphabet, although what it wrote is written every day.