Passatta is a cooked tomato concentrate.
Some passatta recipes have tomatoes cooked with pork, bacon and vegetables and the resultant liquid is kept and used in cooking.
Vegetarian passatta would obviously not include any meat.
Some people just peel and remove the seeds from tomatoes and then put the remaining tomato thorough a food processor to make a puree that can be kept in the fridge for a few weeks.
If the recipe that you are following only uses a small amount of passatta, you can get away with using tomato paste, or puree, that you buy from a supermarket.
You can buy ready-made passatta from supermarkets or good food shops. The passatta will be in the tinned and pureed tomatoes section.