As a noun, 镇 means a town smaller than a city (市) but larger than a village (村). As a verb, 镇定 (zhèn dìng) means to calm down, and 镇压 (zhèn yā) means to suppress/crack down.
Left metal radical — the indexing radical. Original sense of 镇 was 'a heavy metal weight pressing things down'. From the literal weight came 'suppress, hold steady, calm' and 'a fortified strategic point' (a town with military weight). Same family as 钱 (money), 银 (silver), 铁 (iron).
Right 真 — supplies the sound (zhēn → zhèn, only tone shift). Same phonetic series as 慎 (cautious), 填 (fill in), 颠 (top). Pure sound carrier; the 'true/genuine' meaning of 真 isn't directly relevant — though a 'true / steady' weight neatly fits the suppress/calm sense.
