In this restaurant, seasoning is kept to a minimum for you to enjoy the natural taste of the ingredients.
When you want to use the most basic elements of a material, you end up with raw materials. Maybe it's the raw materials you're going to use to build a house. Maybe it's the raw materials you need to make a good soup. Either way, they haven't been touched much (yet).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The reading for 素 is the そ reading, so be careful. You can remember by thinking how you have to sew the building together using these raw materials.