Simple Definition:

Procrastinate means to delay or put off doing something [until a later time] that needs to be done. It often involves avoiding tasks by doing something else instead [like watching TV or checking social media]. For example, if you have a project due tomorrow and you keep telling yourself you will start it later, you are procrastinating.

Very Simple Definition:

Procrastinate means to delay or put off doing something that needs to be done. For example, if you have homework but you keep playing video games instead, you are procrastinating.