Chop is a term in poker that has many different meanings. The meaning being used depends on the context:
Split A Pot
The most common usage is "to split a pot which would normally go to one winner". This is common in games like Hold 'em where the existence of community cards leads to a greater frequency of equal hands.
Chop is not used as a term when a pot is split between high and low in a "hi-lo split" game like Seven-card stud hi-lo. In games like those, players merely talking about "winning high" and "winning low", or "scooping" (winning both). If, however, the high or the low half of the pot is itself divided among more than one player, it could be called "chopping" that portion of the pot, though it is more often referred to as "quartering", for the obvious reason that we're down to a quarter of the total pot now.
Tip A Partial Chip
In games where chips are of a denomination higher than $1 (for instance, a 20/40 game played with $5 chips), players who wish to tip the dealer an amount that is less than one chip will toss the dealer a chip of any particular denomination and ask the dealer to "chop it". The delaer will break the chip into smaller denominations, including one or more $1 chips, will keep one $1 chip, and return the rest to the player as "change" for the tip.
Asking the dealer to chop a tipped chip is implied to mean you want to tip the dealer $1, a standard tip amount. If you wish to tip them more, you simply toss them back some of the change they hand you (or tell them up front "keep $2 of it").
Sacrifice A Small Blind
In California, due to the fixed-fee rake structure imposed by state law, it is often advantageous for the small blind to simply sacrifice their blind if all other players have folded to them. This has come to be known as a "chop", and is considered courteous (and may even be financially to the small blind's advantage -- see the rake page for more details).