In C, there's a data type called a "pointer". Let's say you have a
gigantic object (like, an entire deck of cards) sitting somewhere in
memory, and you don't want to have to make a copy of the object (copy
every single card) when you want to know how many jacks there are. So
instead of copying the entire object and sticking the copy in the
place that your method/function looks for the object, you pass around
an arrow that says "the deck of cards is sitting over there!" . The
arrow is called a pointer. And when you want to look at where "there"
is, and do something to/with the stuff over "there", that's called
"dereferencing a pointer". ("pointer" and "reference" are almost the
same thing)

When I look at the letters 'asdfasdf', I see the shapes of the
letters, but they don't mean anything to me; 'asdfasdf' does not point
to any concept in my head, and so I cannot dereference it.  When I
look at 'Jason', on the other hand, I can dereference the image of the
letters, and replace the image of the letters 'J-a-s-o-n' with my
concept of myself.