These are acts that were performed constantly and regularly by the Holy Prophet Muhammad [s] and his Companions [r]. These acts should not be abandoned without a genuine reason. A person who does so and is persistent in his negligence, will be a sinner. However, if somebody omits it by chance, there is no harm.
SUNNAH GHAYR MU’AKKADAH :
These are acts performed occasionally by the Holy Prophet Muhammad [s] and his Companions [r] , and may be omitted even without a valid reason.