The act of washing was preformed to correct a condition of ritual impurity and restore the state of ritual purity.The ritually impure or ( unclean) person is prohibited from per forming certain functions and participating in certain rites. Ablution, following a with drawal period in and ,in some cases, other special rituals, render him again "clean" and permit performance of those acts which his impurity had prevented.
Ablution must not be confused with washing for the sake of cleanliness.
This is evident from the requirement that the body be entirely clean before ablution (Maim Yad, Mikva'ot 11:1),but there may nevertheless be some symbolic connection. The ablutions, as well as the impure ties which they were deemed to remove, were decreed by biblical law, and understood by the rabbis in religious not hygienic or magical terms. This is shown by R.Johanan b.Zakkai's to his disciples who had questioned an explanation he gave to a non-Jew about ritual purity;"'The dead do not contaminate and the water does not purify.'It is a command (gezeirah) of God and have no right to question it"(Num. R. 19:4).

In the first type of ablution The person or article to be purified must undergo Total immersion in either mayim chayyim ("live water"), i.e. a spring, river, or sea, or a mikveh, which is a body of water at least 40 se'ahs(approx. 120 gallons) that has been brought together by natural means, not drawn. The person or article must enter the water in such a manner that the water comes in contact with the entire area of the surface. According to law one such immersion is sufficient, but three have become customary.

Total immersion is required for most cases of ritual impurity decreed in the Torah. Immersions were required especially of the priests since they had to be in a state of purity in order to participate in the Temple service or eat of "holy" things.

The high priest immersed himself five times during the service of the Day of Atonement. Other individuals had to be ritually pure even to enter the Temple. However, it be came customary among the Pharisees to maintain a state of purity at all times, a fact from which their Hebrew name Perushim ("separated ones") may have developed. (L.Finkelstein, The Pharisees (1962),76ff; R.T.Hereford, The Pharisees(1924),31ff.).

Total immersion also came to form part of the ceremony of conversion to Judaism, although there is a difference of opinion concerning whether it is required for males in addition to circumcision, or in lieu of it Yev. 46a).

Since the destruction of the Temple, or shortly thereafter, the laws of impurity have been in abeyance. The reason being, the ashes of the red heifer, which are indispensable for the purification ritual, are no longer available. This, everybody is now considered ritually impure. The only immersions still prescribed are those of the niddah and the proselyte, because these do not require the ashes of the Red Heifer and because the removal of the impurity concerned is necessary also for other that purely sacral purposes (entry into the Temple area, eating of holy things). The niddah is thereby permitted to have sexual relations and the proselyte is endowed with the full status of the Jew.
The immersion of the
niddah and the proselyte require Kavvanah ("intention") and recitation of the benediction. The Proselyte recites the benediction after the immersion because until then he cannot affirm the part which says"...God of our fathers... who has commanded us"
Since ablution at it's due time is a mitzvah it may be preformed on the Sabbath, but not nowadays on the month of Av or the Day of Atonement. Except for the niddah and the woman after childbirth whose immersion should take place after nightfall, all other immersions take place during the day.

Maimonides decided (Yad, Issurei Bi'ah 13:7), that a proselyte who immersed him self in the presence of only two members of the Bet Din is not a proselyte.