In many ways ,They both can mean understand ,know But 理解 is more like comprehend , to understand somethings in detail and deep way , which is complicated or difficult 了解 mean just have information , know it ,maybe not very understand Hope my explaination will be helplfu .

1) 了解 implies that you just know something a bit , not thoroughly. It's more about getting some information from sth you don't know much. e.g 你要了解一下我们的新产品吗？Do you wanna get some information on our new product? 2) 理解 means you know something thoroughly, it's more about understanding someone's feeling and mental activity. e.g 我能理解你现在的感受。I understand hlw you feel now.

了解：investigate；你去了解清楚这次火灾发生的原因。 理解：nuderstand 你理解我上面那句话吗？

理解 mean deep understanding 了解 mean superficial understanding