Mechanics is your execution of micro and macro. Fundamentally, your mechanics, as a player, represent the degree to which you have bridged the divide between mind and game - that is, your ability, as a player, to do what you want to do.
Builds are pretty meaningless without good mechanics.
A strong build order is useless if performed sloppily. Mechanics means pressing the right buttons at the right time. When you are playing a StarCraft II match you will be managing a lot of things at the same time. Your units and your buildings and your economy and your technology all need input from you, the player, to operate. As you play you should cycle through all of these things and ensure they are doing what you want them to do. This is sometimes known as "going through the motions" or multitasking. A general-purpose short cycle might be:
- Make sure worker units are being produced.
- Make sure combat units are being produced.
- Construct buildings.
- Research upgrades.
- Issue commands to your army.
- Repeat from #1.
Constantly checking up on the status of all aspects of your gameplay will allow you to execute the actions that need to be executed at the exact moment they are needed. The quicker you are able to accurately work through these types of cycles, the more refined your mechanics will be.