This sampling technique is often called accidental sampling and uses participants that just happen to be in a given place at a given time.
For example, imagine a pollster asking opinions of shoppers in a mall. The sample is convenient because they’re already there, but the generalizability is limited because they just happened to be in the mall at a given time. I was shopping with my wife at a mall in Miami a couple of weekends ago; one large department store was conducting a sale aimed at local Hispanic youth and had one of the local radio stations broadcasting in Spanish from within the store. Imagine a newspaper reporter was there asking shoppers their thoughts on ending the war in the Middle East. The results probably wouldn’t represent all shoppers in the other stores in the mall, much less the city in general. The argument could be made that they are generalizable to other Spanish- speaking youth, but are they really? What about the kids who were working, simply not interested, or didn’t have enough money to go shopping that day. Aren’t their opinions important too?