At first, I thought these were worms! These are tomato seedlings growing inside the mothership. The condition is called vivipary - Latin for "live birth." Seeds contain hormones that control their dormancy. This is likely a tomato that was picked weeks ago, and then it sat on the counter in the kitchen. The warmth of the kitchen and moisture inside the tomato caused the seeds to use up all the hormone that controls growth and boom! Babies arrive.
These new plants would eventually poke through the skin of the tomato and keep on growing. You could plant these sprouts and eventually get new tomatoes. They likely would not be just like the tomato they came from, since the seeds were the result of pollination that included another tomato flower and genes from a different plant. I'm not sure if you want to eat these sprouts. In my research, there was conflicting information about whether or not they can cause tummy trouble and I'm not up for that gamble.
By the way, although this commonly happens in tomatoes, it can also happen in an ear of corn, peppers, apples, pears, oranges, lemons, limes, tangerines and avocados.