Most ordinary magnifying devices use a lens.
Lenses are usually made out of glass or plastic,
which are transparent, but those materials have an
important effect on the light that passes through
Have you ever heard that water "bends"
light? If you don't know what I'm talking
about, take a clear glass, fill it with water and
put a pencil in it and notice how the pencil looks
like it's split in two. This is called
"refraction." Light behaves differently in
the water than it does in the air. The same effect
is found in materials like glass and plastic.
A lens has a dish-shape to it, which causes the
light rays entering it to change direction. The
"focal point" is the point in space where
those rays come together and the image you are
looking at comes into focus, and looks bigger (or
smaller if you flip the lens around).