Passatta is a contentrate that is made out of cooked tomatoes.
There are lots of different recipes for passatta. Some recipes have the tomatoes cooked with meat, such as pork and bacon, whereas some have purely vegetarian ingredients such as onion, carrot and celery.
A vegetarian passatta would obviously not include any meat.
Some cooks simply peel and deseed tomatoes and then process them in a food blender to make a puree that can be kept in the fridge for a few weeks until it is needed.
If the recipe that you are following only uses a little passatta, you could just substitute with a tomato paste, or puree, that you buy from a supermarket.
You can buy ready-made passatta from supermarkets or good food stores. You should find passatta in the tinned and pureed tomatoes section.