On the atptour site, next to the player's ATP rank there is the letter T. What does it mean?
If you check the ranking, you can see that this is always shown next to several players - so it means that the players are tied for the same position.
For example, here is one screenshot from ATP website:
The screenshot is taken from the current status of this page: https://www.atptour.com/en/rankings/singles/?rankDate=2019-11-25&countryCode=all&rankRange=1001-1100
This question contains more detail on criteria that are user (beside the points) in the ATP rankings: What tie-breaker is used in the ATP ranking, when two players have the same number of points? (So T next to the ranking means that the players have the same number of points and also the other ranking criteria are the same.)
ATP stands for Association of Tennis Professionals. It basically ranks tennis players on world rank wise.