Tarpaṇa is one of the items connected with āhnika or daily ritual to be performed by a dvija (the twice-born, members of the first three castes). It is a part of snāna (bath) as also the pañcamahāyajñas (See PAÑCAMAHĀYAJÑAS for details.).
While still standing in water after bathing (in a river or tank) one has to offer water thrice taking it in the joined palms of hand, with appropriate mantras, to all beings of creation, from Brahmā (the creator) up to a blade of grass.
Tarpaṇa to devas (gods), pitṛs (manes) and ṛṣis (sages) is a compulsory part of the daily routine of every brahmacārin (vide Manusmṛti 2.176).
The water may sometimes be mixed with sesame. Such tarpaṇa is said to satiate the gods, manes and sages to whom it is offered.