General understanding:
* Naiintindihan (formal) - This is the most common and formal way to say "understand".
* Nauunawaan (formal) - This is a more literary way to say "understand".
* Gets (informal) - This is a slang term that is often used by younger people.
Specific contexts:
* Naintindihan ko (I understand)
* Naiintindihan mo ba? (Do you understand?)
* Pakiulit (Please repeat) - This is used when you didn't understand something.
* Hindi ko maintindihan (I don't understand)
The best way to choose the right word depends on the situation and who you are speaking to.