Simple rules and a strong theme make this game easily accessible to everybody:
Build your own farm!
You need room for yourself and your family, fields for farming and pastures for a lot of animals.
Send out your people to work!
In order to take an action in Agricola, you need to place a person on an action space. This will provide you with grain, allow you to plow fields, and you can even build a wind mill - just like that!
Diversity is key!
Monoculture is boring. Keep a good balance between farming and animal husbandry and you will have a flourishing and nice looking farm in the end, whether you win or not.