Block to size means that you will either wash your sweater pieces and pin them out while wet to the dimensions for the finished sweater pieces and let them dry,
OR you can pin the pieces to the sweater size and spray the heck out of them with a water bottle until thoroughly wet and let them dry,
OR you can do the above, and use a steam iron to wet the fabric and block it into shape. You never want to put your iron directly on the fabric, and I don’t recommend using this method for yarns that have any kind of synthetic materials.
The method I choose for blocking depends on factors like what kind of yarn I used, the type of fabric (for example, for lace, I find wet blocking works best to open up the holes in the fabric), and how heavy it is (for a really bulky sweater, I never wet block because I don’t want to wait two weeks for it to dry!)
Here is a basic article on blocking:
And here are some really good articles on seaming:
And finally one on weaving in ends:
All this finishing work can be kind of a hassle, but doing it carefully really makes a difference in how nicely your garment turns out.
I should also mention that 100% synthetic yarn can’t be blocked in the above ways-you pretty much just want to machine wash and dry it.