I'm hoping someone gives you a more concise answer than I can but know from reading on here that these fruits are poisonous so should not be eaten.

Potatoes and tomatoes are both members of the Solanum family of plants. What you have there are seed fruit heads on your potatoes that have replaced the flowers and they are poisonous. So you need to dig up the spuds and enjoy they have now finished because all the energy that was forming the tubers is now going into the fruits.