An unit of alcohol is a way that grown-ups measure how many drinks they've had. It's like counting how many toys you have or how many cookies you've eaten.
Each unit of alcohol is like a little marker that tells you how much alcohol is in your drink. The bigger the drink or the higher the alcohol content, the more units of alcohol it will have.
For example, a small glass of wine might have one unit of alcohol, but a big glass could have two or even three units of alcohol. A strong beer might have more units of alcohol than a light beer, even if they're the same size. A grown-up can use the number of units of alcohol they've had to decide if they're drinking too much, and if they need to slow down or stop.
Remember, alcohol is only for grown-ups to drink and in moderation. It's important to always stay safe and never drink and drive.