What is the meaning of ѳ?

A letter of the Old Novgorodian alphabet, written in the Old Cyrillic script.

The letter fita (Russian фита́ (fitá)) or ѳита́ (fitá)). A consonantal letter intended to represent the Greek phoneme /θ/, which was approximated in Russian by /f/. Replaced by the letter ф (f) in the spelling reform of 1917.

Source: wiktionary.org