What do we mean when we say it today?
You might recognize the second half Cassius' statement "the fault, dear Brutus, is not in our stars" as the inspiration for the title of the popular book Fault in Our Stars. Author John Green makes a huge change to his bestseller though. Missed it? He deleted the word not from what Cassius says.
Cassius is saying that he and Brutus are in command of what happens to them, or to put it another way, the stars (a.k.a. fate) are not responsible for their lives. Yet Green's novel has no not in it, so the title does blame the stars for whatever happens like getting cancer or dying. See how that changes the meaning altogether?
In our modern society, we put a lot of emphasis on our own decisions (and not on fate), so it's easy to see how Cassius' quote is popular now. You've probably heard at least one of these before: it's up to you. Make your own luck. Make your dreams come true. That's the whole idea behind what Cassius is saying in the first place.