A stencil is a somekind of paper or cardboard from which a specific shape is cut out so to reduce effort to make that picture again.
So like you have to make a stencil of circle
take a cardboard and cut out a circle out of it.
Now that cardboard peice is your stencil
if you now need to draw a circle, you need just a little time
place stencil on paper and colour in cutout part and then you will get a circle on paper without enough effort.Hope it helps