Origin
This phrase, for the most part, is seen in a poem called Hudibras. The first two parts of the poem were written by poet Samuel Butler in the years 1663 and 1664.
Explanation
You shouldn’t assume that something will definitely happen before it really does. Don’t make plans based on predicted results that haven’t occurred yet. This phrase is often shortened to: Don’t count your chickens. Don’t count your chickens before they hatch is an idiom which implies not to think of the future before reaching it. It is not wise to rely on something one is not completely sure of it taking place. You need to first have the resources, have an appropriate income and then plan an exotic holiday and not vice versa. Some people, as soon as they buy a lottery ticket think they are going to win. They begin to make plans as to what they are going to do with the money when they get it. When you tell such a person “not to count your chickens before they hatch” what you are doing is warning him.
Examples
The current Prime Minister seems pretty confident that his party will win the general election in May and get re-elected. However, he shouldn’t count his chickens until they hatch.
Harry: When Liverpool wins the FA Cup, I’m going to have a huge party.
Beth: Don’t count your chickens before they hatch! They haven’t even played the semi-finals yet!