It's blossom end rot. The primary cause is a deficiency of calcium in the fruit, but that is usually caused by something else:
The most common cause is irregular or insufficient water, especially after dark. Because tomato fruits are among the few fruits without pores to evaporate water from, the plant relies on root pressure to force nutrients into the fruit. If the roots are too dry, the pressure is too low to force calcium all the way to the opposite end of the fruit from the stem. Treatment involves watering more often if the plant is in a container, or watering more deeply and mulching, if the plant is in the ground.
A rarer cause is an actual calcium deficiency in the soil, occasionally seen in some cheap commercial potting composts, or in acidic sandy or peaty soil. Cautious application of lime or gypsum is the cure there. Some gardeners like to use calcium nitrate, but that sometimes overdoes the nitrogen, and doesn't last long.