Units are a measurement used to describe positions, speeds, and masses of objects. In the scientific world, scientists all use the same types of units (for example, meters and kilograms) to be able to communicate accurate data to each other without any mix-ups.
Frames of Reference is a fancy name for points of view (POV). If one person has a different POV than another, they will see things differently. One example of different POV's is that of an astronaut in space seeing the earth move, while a person on earth will not notice the rotation of the planet.