Booing means shouting "BOO!" If people shout "boo" after a performance (for example, a concert, play or opera) they are showing that they did not like it. It is the opposite of applause (clapping to show that the performance was good).
Booing and hissing used to be very common, especially at the opera. In the early days of opera (from the 17th to the early 19th century) opera audiences often behaved badly. They went to the opera to chat with their friends, to cheer their favorite singers and to boo the singers they did not like, not to listen carefully to the music.
Booing has recently become less common in Western culture except at sports events because people think it is rude. Audiences today are more likely simply not to clap if they do not like a performance. Slow hand-clapping is also a way of showing sarcastic applause.