Thank you for your question about your tomato problem.  From your photo, it looks like your tomatoes are experiencing blossom end rot.  This is a condition caused by an insufficient supply of calcium to the plant, which is a result of inconsistent water uptake.  Calcium is moved up into the tomato plant through moisture.  To prevent blossom end rot make sure that your plants have  sufficient, regular water.  Also, do not over-fertilize with nitrogen, as this can also play a part.

Here's a good publication by Michigan State University extension that goes into further detail.
https://www.canr.msu.edu/resources/blossom_end_rot_tip_sheet