Formal:
* Je comprends votre point de vue. (I understand your point of view.)
* Je comprends ce que vous voulez dire. (I understand what you mean.)
Informal:
* Je te comprends. (I understand you.)
* Je comprends. (I understand.)
* J'ai compris. (I understood.)
* Je suis d'accord. (I agree.)
More specific:
* Je vois ce que vous voulez dire. (I see what you mean.)
* Je suis d'accord avec vous. (I agree with you.)
* Je sais ce que tu ressens. (I know how you feel.)
Note: In informal situations, you can often use "Je te comprends" even if you're not specifically referring to the person's point of view. It can be a more general expression of empathy.
Choose the phrase that best fits the situation and the level of formality you want to convey.