Einstein found that what you measure for length, time, and mass depends on your motion relative to a chosen frame of reference. Everything is in motion. As you sit in your seat, you are actually in motion around the center of the Earth because of the rapid rotation of the Earth on its axis. The Earth is in motion around the Sun, the Sun is in orbit around the center of our Galaxy, the Galaxy is moving toward a large group of galaxies, etc. When you say something has a velocity, you are measuring its change of position relative to some reference point which may itself be in motion.
All motion is relative to a chosen frame of reference.
Seconds and meters are relative quantities. However, an observer inside the fast-moving object sees everything inside as their normal length and time passes normally, but all of the lengths in the world outside are shrunk and the outside world's clocks are running slow. A shower of very fast-moving muon particles are created very high up in the atmosphere. Their short lifetime should allow them to travel at most 600 meters. Because they are moving close to the speed of light, the muons' internal clocks are running much slower than stationary muons.