what are the lines in a function called? They look like this "y=I3-xI

This is known as an "absolute value" function...in this case the absolute value graph  of  3-x

Here's the graph :  https://www.desmos.com/calculator/iwhqljrm3d

I looked it up:  the SYMBOLS are 'called'   ...  'bars'   

I'm very fond of 'bars'  ......