Cassius is planning to write a petition with signatures from discontented or dissatisfied citizens in reference to Caesar becoming too ambitious. Cassius fabricates a petition. He pretends that the petition is the voice of angry citizens desiring or insisting that Caesar be removed from power.
Cassius sends the petition by Cinna:
Don’t worry. Good Cinna, take this paper,
And see that you lay it on the praetor's chair,
Where Brutus may find it, and throw this
In at his window. Seal this up with wax
On old Brutus' statue.
Cassius writes the petition and signs false signatures. He leaves the petition where Brutus can find it. By doing this, Brutus feels confirmed in his feeling that Caesar is or has become too ambitious. Brutus is concerned with the welfare of Rome, and Cassius knows Brutus will give the citizens of Rome what they desire.
Caesar is right to be aware of Cassius' "lean and hungry look.". Cassius is quite dangerous to Caesar's safety, and he will do anything to convince Brutus to become one of the conspirators.