This is the second of the two Glagolitic signs for the vowel i, and it carries the numeral twenty. Old Church Slavonic seems to have had a single i, so the two letters do not record two sounds, and the distribution between them is a matter of scribal habit rather than of pronunciation.
The habits are worth attending to because they are consistent within a tradition and inconsistent between traditions. In some manuscripts one sign is preferred before a vowel and the other before a consonant. In some, one form opens a word and the other does not. In some the choice avoids a collision of two similar shapes. None of these rules is universal, and an editor working on a newly identified fragment will often use the distribution as a first indication of which family of manuscripts it belongs to.
The numeral twenty follows from the Glagolitic system of counting the letters of the alphabet in sequence. This is the eleventh letter and the second of the tens, so it takes twenty, and the count continues in tens through the middle of the alphabet before moving to hundreds.
Because the sound is common the letter is common, and it is one of the signs a reader of the script meets on every line, in both the rounded hand of the earliest codices and the angular hand of the Croatian tradition.