mkdir makes a directory at each path that you give it:

learner@tidepool:~/shop$ mkdir archive
learner@tidepool:~/shop$ mkdir january february march

The second line makes three directories, next to each other.

The directories above must exist

Without a flag, mkdir makes only the last name in the path:

learner@tidepool:~$ mkdir web/releases/v1
mkdir: cannot create directory 'web/releases/v1': No such file or directory

web and web/releases do not exist, so mkdir stops.

-p: make the full path

The flag -p makes each missing directory in the path, from left to right:

learner@tidepool:~$ mkdir -p web/releases/v1

A directory that already exists