Leviticus 27:33 "It shall not be redeemed"

When a cowboy takes his herd of cows out to graze in the pasture, he counts the heads of his cattle to make sure that he will not lose any of them.

As the cattle moves from place to place, he counts them again to be positive that none stray away. As he leads them back to their barns, the animals are counted once again. The cowboy does all this in order to insure that he will not lose any of his valuable herd.

The Torah encourages us to count our flocks, as well.

HaShem wants us to remember that He is the One who provides us with all our needs and possessions. When we count our herds, every tenth animal (known as the "animal tithe") must be set aside for HaShem.

Those animals are then brought as sacrifices in the Beit HaMikdash (see Positive Mitzvah 78).

Unlike the cowboy who counts his herd so as not to lose any cattle, we count our flocks with thanks, in order to donate a tenth of the flock to the Beit HaMikdash.

This Negative Mitzvah regards those animals counted as a tithe and as sacred to HaShem. They must not be sold or redeemed.