The pipes, which in the shell are represented with the symbol | , are used to join two commands on the terminal, taking the output of the first command and using it as input of the second.
It is common to see the commands grep and ps together, an example below.
Example: lists all processes with your username 
ps aux | grep user
Example: count the lines of a file, using the nl command 
cat example1.txt | nl
    1  Car
    2  Computer
    3  Robot
    4  Smartphone
    5  Videogame
Example: passing expressions to bc to do calculations 
echo "(12 / 2 - 3) * 3 + 1.5" | bc
    10.5
You can use the commands that you learned and put them together, always respecting the logic of the command.