Passata is a medium-thick, pourable, uncooked red tomato sauce made from crushed and strained tomatoes. It has about the same thickness as tomato ketchup.
To make passata, tomatoes are skinned, then passed (thus the name) through sieves to crush the tomatoes and remove the seeds at the same time.
Note that passata isn't cooked -- this isn't a ready to go pasta sauce -- but rather sauced tomatoes ready to go for whatever you need them for.
Italians don't always make every pasta sauce totally from scratch. They often start with a bottle of passata.
If it's a larger amount called for, you can just use tinned tomatoes, whizzed through a blender or food processor then strained.
But never substitute large amounts of tomato paste for large amounts of passata, as the tomato paste will be too rich and sweet.