How to understand their names ?

Scientists number the carbon atoms in fatty acid chains using the omega " " numbering system.

Let's try an example

Alpha linolenic acid is written like this: C18:3 3

It means that linolenic acid has 18 molecules of carbon in its chain and 3 double bonds.
3 means that the first double bond occurs at the third carbon from the methyl group (CH3), and the rest of the double bonds follows after a methylene spacer (CH2).