If a print fails, a part of the print itself or its support structure may have broken off or delaminated. These pieces of cured resin will be floating in the vat and will cause subsequent print failures or damage if not removed.
While you could drain the entire vat and remove these pieces manually, the printers come with a built-in tool called the clean job which exposes the entire build area. This creates a thin layer for the debris to stick to that can then easily be peeled off.
Use the tamper tool and a sacrificial strip of copy paper or plastic mesh fabric under the tool to ensure ease of removal of the cured layer without damaging the membrane or LCD. Don't place the tool in the center of the exposed area, the cured layer should be peeled from the edge.