Tolerance is the reduction of the effectiveness of a drug after a period of prolonged or heavy use of that drug or a related drug (cross-tolerance). There are two types of tolerance we should touch on concerning alcohol, blood alcohol concentration and impairment.
The first is metabolic tolerance in which the alcohol is metabolized at a higher rate (up to 72% more quickly) in chronic users. Remember, as a sort of self-preservation defense mechanism, over time heavy drinkers can become super efficient and metabolizing and eliminating alcohol from the body. Therefore, since the consistently heavy drinkers are eliminating alcohol more quickly than the average person, the peak blood alcohol concentrations will be lower than for an average drinker who consumed the same amount of alcohol. Stated another way, since heavy drinkers burn alcohol faster they have to drink faster to reach a given blood alcohol concentration and get drunk.
The second is acquired or behavioral tolerance which refers to a person's ability to mask the impairing effects of alcohol. Essentially, over time people who are consistently exposed to a given amount of alcohol can learn to behave in a way that makes it far more difficult to realize they have a high blood alcohol concentration. For example, assume that on a daily or near-daily basis a person consumes enough alcohol to reach a blood alcohol concentration of 0.15. Over time, that person will learn to mask the impairing effects of the alcohol - essentially the person will become a "better drunk" - and it will be less obvious that the person's BAC is above the legal limit.