What does the .ros folder contain ?

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What does the .ros folder contain ?

It's the default location for all sorts of "system data", meaning log files, rospack caches, rosdep caches and a few more types of files used by some core parts of ROS (in ROS 1 at least).

.ros is also the default working directory when nodes are started using roslaunch, which causes quite some confusion as that would be where ROS nodes would try to read/write files specified using relative paths.

I believe this is some kind of cache (Please correct me if I'm wrong)

most of the space will likely be taken up by log files, not the caches.

Is it safe to delete that folder

there'd be no need to delete it. If you'd like to reduce the amount of space it occupies and don't care to retain the logs, just run rosclean purge and answer the prompt. See wiki/rosclean for some more info on that utility.