A birth plan is a document that tells your healthcare providers what your preferences for labor, delivery and newborn care are. You can pull check off lists from the internet. Two different ones I have found are here and here. There are many out there and sometimes the hospital you are delivering at will give you a generic one to fill out if you so desire.
You can also write your own after deciding what your preferences are. If you decide to write your own be gracious in how you write it.
This document is not legally binding and will not hold up in court if something is done outside of your birth plan.
It is up to you if you want to write one or not. Some people find this helpful to communicate what their expectations are to others. You may find it easier to stick to your plan if it is written down. This is not needed, but can be beneficial.
Do you write out a birth plan?