Urine flow rate is a measure of the quantity of urine

Urine

Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

 produced in a specified period of time. Common periods of time used include "minute" and "24 hours".

It can be used in the calculation of clearance

Clearance (medicine)

In medicine, the clearance is a measurement of the renal excretion ability. Although clearance may also involve other organs than the kidney, it is almost synonymous with renal clearance or renal plasma clearance. Each substance has a specific clearance that depends on its filtration characteristics...

 and glomerular filtration rate.

The letters "V" (for volume) and "Q" are both used as a symbol for urine flow rate.

The average rate changes with age.