CRS stands for Coordinates Reference System and has been set up to accurately define positions on the surface of the Earth as coordinates.
The most well-known CRS is WGS84 which defines locations as a pair of latitude/longitude.
Thousands of CRS are defined with different naming conventions, the most used being EPSG:XXXXX (e.g. EPSG:4326 is equivalent to WGS84).
When dealing with geospatial data, the pain point often requires converting coordinates from a CRS to another.
To help with that there are plenty of tools and libraries out there, enjoy the links below.