P0 label indicates critical bug, regression or failure, usually in production or in a release candidate. It means the assigned developer should stop working on all current tasks and focus on P0 until it's fixed/resolved. Normally there should be zero open P0 bugs.
P1 indicates crucial bug or feature, blocking current sprint release. No new tasks should be started until P1 resolved.
P2 indicates important problem that we want to resolve as soon as possible.
P3 indicates a time-permitted task: minor/rare bug, non-critical request, research, documentation update, ongoing discussion etc.