This proverb first appears in English in a book of proverbs assembled by Thomas Heywood in 1546:

Thus by these lessons ye may learne good cheape
In wedding and all things to looke ere ye leaped

The proverb in this case addresses young women, warning them to really know what they are getting into before they marry. With divorce not allowed in England at that period except in very rare cases, it was important that a woman carefully check out a potential future spouse before making a terrible mistake.

The proverb, however, goes back to a fable by Aesop. In it, a fox trapped in a well persuades a goat to jump in and save him. The foolish goat leaps in without looking. The fox is able to climb on the goat to get out, leaving the goat trapped in the well.

In both cases, the proverb means that one should check out a situation carefully before getting involved in it so as to not get hurt.

Proverbs try to give us a message without explicitly saying what it means and this one is no exception. While "look before you leap" can be literal in some situations this really means to think about what you are going to do before you actually do it. Considering the consequences or outcomes of our actions may result in a person making a different, and perhaps wiser, decision. Many decisions we regret often come from choices we made without enough forethought so they were made in haste. So this proverb reminds us to think before we act.