"High beam" is the term used to describe
erect nipples, usually those belonging to a
woman.
In Australia, the term "high beam" is usually used by males, and only in 'appropriate' circumstances such as nightclubs, pubs and parties (ie, where alcohol is involved).
If a woman walks past a group of high-spirited males whilst her nipples are erect (due to the chilly temperature or another factor, such as sexual arousement), it is common for at least one of the men to say "she's high beaming us!" or "turn off the high beam!", much to the amusement of the other males.
"High beam" is derived from the phrase of the same terminology which means that a car or other vehicle has its headlights on to full effect, thus the basis of the metaphor.