lack of understanding; lack of sympathy
Refers to a state of not understanding someone's feelings, situation, or ideas; often implies a failure to empathize or appreciate another's perspective. Used both as a noun and as a na-adjective.
I was really disappointed by his lack of understanding.
Society's lack of understanding is making the problem worse.
無関心 means indifference or apathy, a lack of interest, whereas 無理解 is specifically about not understanding, which may coexist with or lead to indifference.
Compound of 無 (mu, 'without') and 理解 (rikai, 'understanding'). The word follows a common pattern of forming nouns meaning 'lack of X'.