Here's an example:
Julie goes out to the pub with her friends and over the course of the evening, she drinks three regular 175ml pub glasses of red wine. Each contains about two units of alcohol. As she drinks, the alcohol is absorbed through her stomach wall and into her blood where it waits until her liver can deal with it.
Julie's liver can only process about one unit of alcohol per hour. The exact length of time it takes depends on the amount of enzymes in her liver and how healthy it is, and this varies from person to person. Julie's liver will process 95% of the alcohol and a tiny amount of alcohol will escape from her body through her breath and urine.
Julie has been in the pub for two hours and has finished her third glass of wine, which amounts to six units. At this stage she is becoming merry and she may also be a bit clumsy and acting on impulse. Since Julie's liver can only process one unit per hour, her body has absorbed all six units but her liver has only processed two.
Until Julie's liver can deal with it, her blood spreads the remaining four units of alcohol throughout her body, affecting her brain and other tissues. It will take another four hours before her liver will finish processing the rest of the alcohol.
There are other factors involved in how fast Julie's body absorbed the alcohol. She didn't have a full meal before going out drinking. If she'd had a balanced meal, her body would have absorbed the wine more slowly than when drinking it on an empty stomach.
Julie's body is absorbing the alcohol faster than her liver can process it. Since the processing part is slow, Julie needs to pace her drinking to prevent it building up in her body.
If Julie drinks any more, she is putting herself and her body at risk.