The first is casual - what you might say to a classmate after a difficult lecture or presentation. The second means the same, but is less casual. If the presenter himself is asking you, it could be more critical, as Anna suggests, implying that you should have followed it, but this interpretation will depend on the tone of voice in which it is asked.

They mean the same thing but the tone of the second sentence is harsher, indicating that the speaker doesn’t expect the listener to understand anything.