UVB is an invisible ultraviolet spectrum of light. It is essential to all living animals, but especially to diurnal reptiles (definition: reptiles active during the day). Nocturnal reptiles also benefit from UVB, but it’s not as crucial. UVB light allows the body to synthesize vitamin D3, which is required to metabolize calcium. Without it, reptiles are unable to use the calcium in their diet and begin to take it from their bones which causes medical issues with their bones and nervous system and will eventually lead to death.
Do the research for the specific animal that you have, but a good rule of thumb is to program the UVB and daytime light/heat to be on for 12 hours and then turned off for 12 hours. During the 12 hours when bright lights are off, there should still be nighttime heat (black or red bulbs, heat mat, or ceramic heat emitter), this is also a time when the animal can get some rest without bright light distracting them.