"I understood what you just said."

Translation: J'ai compris ce que tu viens de dire.

[moonswooning]

Why do you need 'ce que' and not just que?
And why 'de dire', is it because of the viens?

[georgeoftruth]

"Ce que" is indefinite. It refers to an unstated or unspecified thing or idea.

• J'ai compris que tu viens de dire quelque chose. = I understood (the fact) that you just said something.
• J'ai compris ce que tu viens de dire. = I understood what you just said.

As for the recent past, (to have just done something), the construction is

• conjugated venir + de + infinitive

[Kevin968039]

And viens = "come" ? Is this correct?

[georgeoftruth]

Viens does not mean “come” in this case. Viens de + verb means to have just done something. So “tu viens de dire quelque chose” means “you just said something.”

[JasonSnipe1]

thank you georgeoftruth