Do you know what I'm saying?

Do you understand?

Are you following me?

Are you with me (so far)?

Have you got it?

Any questions?

Got it?

Showing Understanding
I see.

I understand.

I get it./I got it.

Gotcha. (Informal)

Expressing Lack of Understanding
I don't get it.

(I'm sorry.) I don't understand.

What do you mean?

I'm not following you.

I don't quite follow you.

I'm not sure I get what you mean.

What was that again?