What does ":" do when used in export env variable?

I am just trying to following this post.

I have replaced "/some/directory/" by "/usr/lib/x86_64_linux-gnu/", resulting in

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib/x86_64_linux-gnu/

Now when I type

printenv LD_LIBRARY_PATH

it prints

:/usr/lib/x86_64-linux-gnu/

I wonder if the : is correct, and if not what I have to do differently.

I am confused why the author used

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/some/directory/

What does the "$" and the ":" do?