Dictionary.com defines "Brand" as "a kind or variety of something distinguished by some distinctive characteristic" and "ambassador" as "an authorized messenger or representative".
Our actions provide direct stimulas to our brand. We are brand ambassadors of our own brand in addition to being ambassadors to the brands we get associated with. It could be our school, college or company. Each individual bears the burden of building or destroying a brand. Among other reasons this is one reason top companies ask employees not to wear company logo T-Shirts to bars and pubs.
The other day, I was coming out of a parking lot. In front of me was a a car displaying a b-school alumnus sticker of a school based near IFFCO in Gurgaon. The driver ahead of me threw his parking permit on the ground instead of handing it over to the guard standing to collect permits. On top of this, he drove away fast giving very little time to the guard to sway out of car's path. I was surprised to see this action from an educated person let alone from a decent b-school. This incident got me thinking about what he had learned in the school and how he was tarnishing his school and himself. It was clear that he did not care about himself or his reputation but he was indeed giving a very negative impression of other things that he was associated to.