This letter stands eleventh, writes the vowel i, and carries the numeral 10 from Greek iota. It is the second of the two Cyrillic signs for i, and the Russian tradition names it the decimal i after its numeral value, against the octal i that precedes it.
Its shape is Greek iota, a single vertical stroke, and in Slavonic it usually carries two dots above it in the printed church books, which is what distinguishes it on the page from the other tall narrow letters around it. The dots are a printers convention and are not consistent in the manuscripts.
Its numeral use is heavier than its use as a sound. Ten is the base of the whole tens series, so this letter sits under a titlo in every number from ten to nineteen, and the Slavonic habit of writing the teens with the unit before the ten means it appears second rather than first in all of them.
It survived in Russian orthography until the reform of 1918, in Bulgarian until 1945, and it is still in use today in Ukrainian and Belarusian, where it writes a distinct vowel and is a working letter rather than a doublet. That is a recurring pattern in this alphabet: a letter abolished as redundant in one language can be indispensable in the language next door, because the sound systems diverged long before the orthographies did.