What "configurable" means in Apple
This term configurable refers to the fact that when you order a Mac from Apple you can choose to configure it with more RAM. You cannot do this after purchase yourself, as the RAM is soldered on during the manufacturing process.

Other configurable options when buying a Mac usually include the size and/or type of storage, the processor you wish to have, bundled software, and so on. The choices you get also depend on the model of Mac you purchase.