A block is a basic shape on which other patterns are based.
They are sometimes also called slopers, although purists will argue that a block and a sloper are different things.
Blocks are typically not made as-is but rather serve as a basis for other patterns so the instructions below will not go in depth about closures or finishes and are for the default Titan block.
Keep an eye out for anything you keep doing whilst wearing the mock-up, are you pulling it down? Constantly adjusting the shoulder? etc. Things like these are signs of where the pattern may need adjusting.
Sometimes you may need to wear the mock-up for an extended amount of time to get a better sense of the fit so don't be afraid to walk around in it for a couple of hours.
It is best practice to make a paper pattern from the mock-up if you have made any alterations, as this will allow you to clean up any lines but also means you have a pattern that you can keep producing garments from.