Someone has hurt the speaker deeply, and the poem describes it in a striking image: sharp tears that stab, and wounds so severe the speaker bleeds rivers. Normally that would mean agony. But the speaker says they felt nothing, because they were simply unable to feel pain.
The final two lines are where the poem turns. Being unable to feel pain is called both a "greatest strength" and a "worst weakness," and the poem leaves the two ideas side by side without explaining them. Judging by the lines, the numbness protects the speaker from suffering, but it may also cut them off from feeling anything fully, including the hurt that would show how much something mattered. The title, a Latin word that seems to mean weakness or frailty, fits that tension.
The mood is cold, quiet, and a little eerie. The short, flat sentences sound calm, yet the bleeding rivers suggest something is badly wrong underneath.