Pitting oedema is swelling that holds a dent: press a thumb firmly into it for about ten seconds, lift, and a pit remains that slowly refills. The dent means free fluid is sitting in the tissue — which both venous disease and early lymphoedema produce, so pitting tells you fluid, not yet which cause.
The reading that matters is the pattern around it: both ankles by evening, gone by morning, leans venous; one limb, including a puffy foot, leans lymphatic; swelling that stops pitting over time in a still-large limb suggests long-standing lymphoedema where the tissue has hardened — firmness is information, not reassurance. The counterpart worth knowing: lipoedema typically does not pit, because the volume is fat, not fluid.
The full how-to — sock marks, the ten-second test, when each pattern needs assessment — is in swelling that pits, and Stemmer's sign is the companion check.