who tells brutus "thou shalt see me at philippi"? what is meant by this?

Brutus was the most important of the conspirators who murdered Caesar. (Without Brutus' support the other conspirators would probably have been too scared to act).

When Caesar's ghost appears to Brutus at his tent he comes with the message:

thou shalt see me at Philippi.

Since Brutus will be killed in the battle with Octavian's army at Philippi, this is correct.