Metabolic (or pharmokinetic) tolerance is a reference to the absorption, distribution, metabolism, and excretion of drugs. Essentially this is how the body processes the intake, use, and excretion of drugs. When a substance is used for an extended period of time the body creates a tolerance that makes the substance less effective, requiring more of the substance to create the same physical effect.
This is why prescription medications sometimes need to be changed or adjusted as time goes by.
A person can easily build up a tolerance to recreational drugs requiring greater doses over time to approximate the original experience. This can lead to overdosing and death. Some substances are addictive and become necessary for 'normal' body functions requiring medical intervention for safe withdrawal.