This line occurs in Act I, scene iv as Mercutio, Benvolio and Romeo are preparing to go, uninvited, to Capulet's feast. The have, as was a custom of the day, entered masked, and are preparing to show up for free food, drink, and dancing with the ladies at their table of the Capulets.
The line you quote appears in context like this:
We'll have no Cupid hoodwink'd with a scarf,
Bearing a Tartar's painted bow of lath,
Scaring the ladies like a crowkeeper,
Nor no without-book prologue, faintly spoke
After the prompter, for our entrance.
But let them measure us by what they will,
We'll measure them a measure and be gone.
What Benvolio is doing in the first part of this speech is commenting on how there won't be a great to-do over their entrance, no pomp and circumstance, because they are, in effect, uninvited guests.
In the final two lines, he's playing with the different meanings of the word "measure." In paraphrase, Benvolio says, "Let them judge us ("measure us") however they want, but we're going to give them (""measure them") a dance ("measure") and then leave."
For more on this scene, please follow the links below.