The at command is used mostly for ad hoc scheduling. The syntax for at is very close to our natural language. This is easiest explained with an example, as follows:

reader@ubuntu:~/scripts/chapter_14$ date
Sat Nov 24 11:50:12 UTC 2018
reader@ubuntu:~/scripts/chapter_14$ at 11:51
warning: commands will be executed using /bin/sh
at> wall "Hello readers!"
at> <EOT>
job 6 at Sat Nov 24 11:51:00 2018
reader@ubuntu:~/scripts/chapter_14$ date
Sat Nov 24 11:50:31 UTC 2018

Broadcast message from reader@ubuntu (somewhere) (Sat Nov 24 11:51:00 2018):

Hello readers!

reader@ubuntu:~/scripts/chapter_14$ date
Sat Nov 24 11:51:02 UTC 2018

In essence, you're telling the system: at <timestamp>, do something. When you enter the at 11:51 command, you will be placed in an interactive prompt that will allow you to enter the commands you want executed. After that, you exit the prompt with Ctrl + D; if you use Ctrl + C, the job will not be saved! For reference, we use a simple command here, wall, which allows you to broadcast a message to everyone that is logged in to the server at that time.