Origin
The first recorded use of this precise phrase was in 1545.
Explanation
Those who precede others will have their needs attended to earliest. Something that you say which means that the people who ask for something first will be the ones who get it, when there is not enough for everyone. It also describes a process of dealing with people using the principle “first come, first served”. It is a service policy whereby the requests of customers or clients are attended to in the order that they arrived, without other biases or preferences.
Examples
We’ve got ten cheap computers on offer. It’s first come, first served.
We should get to the book sale as soon as they open; it’s first come, first served.
You can’t reserve a seat at the movie theatre; it’s strictly first come, first served.