In general, a mask usually symbolizes concealment, which is true for the masks in Romeo and Juliet, but Shakespeare goes a bit further with it too.
We see masks used at Capulet's ball in the first act, as his ball is a masquerade. Not only that, nearly every important person in Verona is present at the ball, concealed in masks. Since they are at a ball, they are behaving gaily while they laugh, talk, dance, and eat. However, what's interesting is that due to the Capulet and Montague feud, Verona is not really a very happy place. It's full of violence and animosity. In addition, it is not just the Capulet and Montague family members that are involved in the feud. As we see in the first scene, their feud is creating war amongst all the citizens in Verona. Whenever the Capulets and Montagues publicly engage in their battle, all of the citizens join in as well, as we especially learn from the Prince's speech:
Three civil brawls, bred of an airy word
By thee, old Capulet, and Montague,
Have thrice disturb'd the quiet of our streets. (I.i.85-87)
We even further see it clearly in Shakespeare's stage directions in which he describes officers and citizens joining in on the street brawl. Hence, since all the citizens of Verona are at odds with each other due to the Capulets' and Montagues' hatred for each other, we see that the happy, gay party masks they are wearing at the ball is really just a facade. The truth is that Verona is not what it is appearing to be at this moment; it is actually full of hatred and violence.
When Romeo wears a mask to the ball, it not only symbolizes concealment, but also misjudgement. Romeo is apparently wearing a mask that is a lot like a clown mask, which we can see from Tybalt's phrase "antic face," meaning comic mask (I.v.58). When Tybalt figures out that Romeo is present at the ball, he is not just furious that Romeo, a Montague, has crashed the ball, he is furious because it seems to him that Romeo has come to mock their ball. Tybalt feels this way based on Romeo's comic mask. He sees the laughing, mocking face Romeo is wearing as his mask and assumes that underneath the mask, Romeo is acting in mockery as well, as we see in Tybalt's lines:
What, dares the slave
Come hither, cover'd with an antic face,
To fleer and scorn at our solemnity? (I.v.57-59)
Hence, based on the mask Romeo is wearing, Tybalt has misjudged Romeo. But even if Romeo was not wearing a mask, Tybalt still would have misjudged Romeo because the true Romeo is hidden behind all of the hatred and prejudice that the longstanding family feud has caused. Therefore, it is the feud that is concealing the true nature of both the Capulets and Montagues, showing us that the mask not only symbolizes concealment but also misjudgement due to concealment.
The mask serves as a symbol of disguise for Romeo to enter the Capulet ball, as the Capulets and Montagues are sworn enemies. The mask serves as a device for Romeo to meet Juliet at the Capulet ball, to which he is not invited. It also serves as a symbol of liberation as under this hidden face Romeo and Juliet may flirt with each other freely as their true faces are not exposed.