Can you explain what it means to be above the legal limit for drinking?

Blood alcohol level. If 18 or over, elevated is considered .08 or greater (that is .08gm/dl or 80 mg/dl). If under 18, an elevated level can be considered as low as .02. Value must be evaluated in terms of dilution, (amount of alcohol per a specified volume of blood). This of course varies depending upon the proof of the beverage (proof is approximately 2x the ethanol (alcohol) percentage).
You're drunk! If you do not drink regularly and therefore have developed a 'tolerance' to a higher blood alcohl level (which is clearly not a reason to commence a path to alcoholism, then being abov eth 'legal limit' generally means you're drunk. Your judgement is impaired...Etc etc.