Bean-to-bar is when people make chocolate from scratch, starting with raw cocoa beans.
Imagine if you want to make a cake. One option is to buy a cake mix where you just add a few ingredients and bake it in the oven. Another option is to buy all the ingredients separately and make the cake from scratch.
Bean-to-bar is like making a cake from scratch. Instead of buying chocolate that has been already made with cocoa powder, someone who practices bean-to-bar buys raw cocoa beans and roasts them. Then they crush the beans to get cocoa nibs, which are small pieces of cocoa. Finally, they grind these nibs into a paste called chocolate liquor.
From there, they will add sugar and other ingredients to make the chocolate taste the way they want it to taste. They will then pour the chocolate into molds and once it has hardened, they have a bar of chocolate they created themselves.
It's a lot of work, but it can be really rewarding to create something from scratch that tastes delicious.