同 is built from the 冂 (open box / border) and 一 components.
It belongs to a phonetic family with 銅, 洞, 筒, and 胴 — they share the 同 component and the reading ドウ.
A common word using 同 is 同じ (おなじ), meaning “same, identical”.
Don't confuse it with 洞, 銅, 筒, and 胴.