The lines in question form part of a long speech by Cassius that is intended to raise doubts in Brutus' mind about Caesar's motives and his fitness for supreme command. Earlier in the speech, Cassius has earlier pointed out some of Caesar's physical weaknesses and infirmities, to underline that Caesar is no better a man than they themselves are:
....I did hear him groan.
Ay, and that tongue of his that bade the Romans
Mark him and write his speeches in their books,
Alas, it cried, “Give me some drink, Titinius,”
As a sick girl. Ye gods! It doth amaze me
A man of such a feeble temper should
So get the start of the majestic world
And bear the palm alone.
He then argues that Brutus himself is in no way inferior to Caesar ("what should be in that Caesar?'), implying that Caesar's rise beyond Brutus' rank has something odd or sinister about it:
Now, in the names of all the gods at once,
Upon what meat doth this our Caesar feed
That he is grown so great?
Finally, Cassius rounds out his oration by invoking historical precedent -- the traditional hatred of the Romans for one-man rule -- and the involvement of a Brutus long ago in the struggle to overthrow the ancient kings of Rome and establish the Roman republic:
O, you and I have heard our fathers say
There was a Brutus once that would have brook'd
The eternal devil to keep his state in Rome
As easily as a king.
In short, in this passage Cassius is saying to Brutus that Caesar is an equal of theirs, or perhaps even an inferior, who has risen above them by questionable means and who does not deserve his high position -- and who may need to be removed from that position by violence.