A Massclaim is when all living players claim their roles at the same time and reveal any night actions they have done throughout the game. By analyzing the roles and night actions, players can then determine who may be lying about their role, or what roles don't seem to make sense or otherwise fit in the game. Generally people are also expected to massclaim their flavor to rule out flavor inconsistencies (which imply scum with a bad fakeclaim, especially in a flavor-heavy game).
Massclaims are usually saved for the end game. To prevent against massclaims being too strong, mods will often include odd roles or give the scum fakeclaims, which have the potential to make a massclaim useless for the town. Likewise massclaiming too early can be beneficial to scum because it can tell them who they need to nightkill and in what order, whereas the town is often far less efficient.