The apparatus
Four pieces of laboratory apparatus are needed to prepare a solution of an exact concentration. Each of them is there to stop a particular kind of error.
Figure to be added
Volumetric flask
A flask marked with one line at one exact volume, chosen to match the volume of solution being made. Flat-bottomed with a long narrow neck, and it takes a stopper.
Wash bottle
A squeeze bottle with a fine jet, used to rinse every last trace of solute off the watch glass and the funnel and into the flask.
Watch glass
A shallow round dish that the solute is weighed out onto. Being able to weigh on it and then rinse it clean is what makes the whole mass transferable.
Funnel
Held in the neck of the volumetric flask so that solid can be washed down into it without any being lost around the outside of the narrow neck.
Why the neck is so narrow
The mark on a volumetric flask sits on the narrow part of the neck rather than on the wide body, and that is deliberate. In a narrow tube a small error in where you judge the liquid level to be makes only a tiny error in volume. On a wide body the same misjudgement would be several millilitres out.
This is not a beaker job
A beaker has graduations, but they are approximate and its sides are wide. Making a standard solution in one throws away the accuracy the careful weighing was for.