We are infinitesimally small and insignificant. I felt the scale of space and the unforgiving progress of time, as characters dealt with the emotional impact of time dilation on them, their colleagues, and family. And we, as viewers, get to experience it as we see minutes pass from our POV only to return to Endurance and realize Romilly has aged over 20 years in orbit, all alone; and Cooper’s kids have suddenly grown up without us noticing.
And as others have noted about the film before, it also relates to the phenomena we experience as we age and our perception of time accelerates. If you’re a parent, you feel yourself slipping farther and faster away from your children with each passing year. Chances are if you watched Interstellar as a parent, you felt the film deeply on another level.