懂

Left radical 忄 is the side-form of 心 (heart). It marks 懂 as a mental act: understanding happens in the heart-mind, not the head, in classical Chinese psychology. Same heart-side radical anchors 怕 (fear), 忙 (busy), 快 (happy), 慢 (slow) — the family of internal states and feelings.

Right phonetic 董 supplies the sound — dǒng matches directly with no shift. 董 itself means 'to oversee, direct', which carries faint semantic flavour: to 懂 something is to oversee it mentally, get a grip on it. 董 is also a common surname (董事长 'chairman' literally 'oversight elder'). Same phonetic in 懂 alone but 董 itself is widely seen.