Untreated the cancer progresses through the stages, ending up at stage 4
which is metastasis and generally incurable.  One hope to arrest the
cancer at whatever stage it is at by surgery etc.  The lines between the
stages are not very clear, so there are substages and detailed lists of
signs for staging, which differ according to the type of cancer, but my
understanding is roughly this.

Stage 4 is detectably metastasised, stage 3 is likely to metastasise but
not detected, stage 2 is well developed but not yet likely to
metastasise, stage 1 is close to the edge of detectability. (and stage 0
is not yet cancer but showing pre-cancerous changes.)