The Poincaré Group consists of 10 mathematical transformations that can be applied to different coordinates of the space-time graph (x, y, z, ct), but do not affect the "spacetime interval". The "spacetime interval" is roughly the 4-D equivalent of 3-D "distance".
The Poincaré Group is a "symmetry group" because the spacetime interval remains invariant.
These transformations are:
Spatial translations:
Time Translations:
Spatial Rotations:
Lorentzian Transformations: