This rule can be used if falling to 0 Hit Points causes unconsciousness rather than death.
If a halfling loses enough Hit Points that they would fall unconscious but not die, they may make a Saving Throw. Success means that they stay conscious until the end of the combat, or out of combat for ten minutes. If they receive further non-fatal wounds within that period they do not need to make further saves.
"There is a seed of courage hidden (often deeply, it is true) in the heart of the fattest and most timid hobbit, waiting for some final and desperate danger to make it grow."
Lord of the Rings.