You may not be used to peers judging your work, but their feedback can be a very useful way to improve your understanding of a certain area. This can happen in different ways:
Peer assessment in a group
As part of a group, you may need to critique or comment on the work that another member is doing. This is necessary in order to improve the quality of the group's work, but it is important that you do this in a sensitive and constructive manner. Group members should listen to alternative points of view, and be open to suggestions about how something could be done differently. A group is not a collection of competing individuals, and shutting down or intimidating other group members will be detrimental to the entire group.
Some group members may be quieter than others, but may have important contributions to make. It is up to all members to nurture an environment in which everyone feels able to contribute.
Formal peer assessment
You may be asked to judge the performance of your peers more formally. This needs to be done as objectively and responsibly as possible. Usually you will be given some form of mark sheet to guide you on what qualities you are looking for and how you might judge these. You may even be called upon to help construct this mark sheet yourselves.