The bottom of the tomato is the blossom end.
That is probably blossom-end rot (BER). It is a problem that could ruin a few tomatoes, but is not a systemic disease that will keep your plants from also producing good tomatoes.
BER is best handled through cultural practices.
1. Any fruits showing this problem, remove and discard them.
2. It is often caused by uneven watering, too little water at some times, too much at other times; try to keep the plants more evenly watered. My understanding is that the uneven watering can unevenly distribute nutrients, including calcium, throughout the plant and fruit.
3. Providing extra calcium can help. Lime the soil before the growing season, or provide gypsum, or calcium nitrate; aim for a neutral soil pH near 7 +/- 0.5.
4. Some tomato varieties resist BER better than others. Flat-bottomed tomatoes seem to have BER more often, those with more pointed bottoms have less frequent BER.