Dzelo writes an affricate, a dz sound made as a single unit rather than as a d followed by a z, and it carries the numeral eight. It belongs to the group of Slavic letters that arose from the softening of an original velar, and in the earliest Slavonic it was a sound in its own right, kept apart from plain z in writing and presumably in speech.
The distinction did not last in most of the Slavic world. The affricate fell together with z in the great majority of the languages, and once that had happened scribes copying older texts had no reason to keep the two letters apart and began to use them interchangeably. A manuscript that maintains the distinction is following an older norm than one that does not, which makes the letter one of the more useful diagnostics in the canon of early Slavonic writing.
Its name is the Slavonic word dzelo, meaning very or exceedingly, an adverb rather than a noun or a verb form, and it continues the recited alphabet.
Where the sound survived, the letter survived with it. Macedonian retains a distinct dz and writes it with the descendant of this sign, so the letter is not a purely historical curiosity but a working part of a modern orthography, which is not true of most of the signs at this end of the Slavonic alphabet.