Material Expressions and Material Functions are key to getting what you want out of the Material Editor.
No matter what type of Material you are building, the Material will always be made up of a combination of Material Expressions and or Material Functions.
Keep in mind that when adjusting Material Functions that are currently in use, you can run the risk of breaking other Materials and Material Instances that use that Material Function.
Also if you are unsure what a specific Material Expression node does, you can always do a quick internet search for the name of the Material Expressions node.
Because most of the Material Expressions nodes are pure HLSL code, node just in node forum, the official Microsoft documentation on HLSL can sometimes help you figure out what the Material Expressions might do.