徐 is built from the 彳 (step / going) and 余 components.
It belongs to a phonetic family with 除 and 叙 — they share the 余 component and the reading ジョ.
A common word using 徐 is 徐々に (じょじょに), meaning “slowly, little by little”.
Don't confuse it with 除, 叙, 余, and 途.