Miso is a salty, fermented soybean paste popular in Japanese cooking because it's filled with glutamate, an amino acid that gives the paste the savory, meaty flavor known as umami.
It can make countless foods taste better, including pasta. Miso is an excellent shortcut ingredient for pasta sauce because its earthy, mellow flavor pairs well with tomatoes.
To get some of miso's umami goodness into your homemade pasta sauce, just melt a tablespoon or two into the pot at the beginning, right as you're sweating the onions.
If you find that the sauce still needs something, or you're working with store-bought sauce, dissolve a little more miso into some water or stock and stir it into the sauce.