Nothing to really worry about. It's a deformation called 'catfacing'. Although the skin and some of the surrounding flesh can be a little tough, it's mostly just a cosmetic issue. The rest of the fruit is just fine. It's not a disease, and it's thought to be caused by a number of factors during the onset of pollination and fruit formation. The most common cause seems to be very cool temperatures during pollination.

Because of its link to cool temps, it's most prevalent early in the season, and usually only occurs on larger-type tomatoes.

I didn't have an answer for what was causing it but it is definitely messing with my trypophobia. haha!