The answer to this question can be found in the first chapter of this excellent novel. We are told that Winston Smith, like other party members, all wear the same uniform which consists of blue overalls. What is significant about this uniform, as is the case with any form of uniform, that it visibly sets party members aside from other groups in society, such as the Proles, because they are readily recognisable and identified as being Party members rather than anything else. In addition, you might like to think about the way in which having one uniform for all party members suppresses their own personal identity and ensures that the bigger identity of the Party is the dominant identity that defines who they are. We are presented with a totalitarian world where those who live in it are given little if any freedom of choice, and the uniform servers to underscore this bleak reality.
In 1984, Party members wear a uniform consisting of blue overalls, as described in Part One, Chapter One. The purpose of this uniform is two-fold: on the one hand, it helps to create a sense of unity between Party members since everybody looks exactly the same. In addition, by wearing a uniform, it is easy to both recognize other Party members and to distinguish Party members from other social groups, like the Proles, who do not have to wear a uniform.
On the other hand, making people wear a uniform is also about exerting control. It prevents people from wearing their own clothes and, thus, from expressing their own style and personality. As a result, the uniform functions as a symbol of social control, much like Newspeak and the telescreens.