Original: Blowing a raspberry or strawberry or making a Bronx cheer is to make a noise signifying derision (and/or silliness), made by sticking out the tongue between the lips and blowing to make a sound reminiscent of flatulence.
Simplified: Blowing a raspberry or making a Bronx cheer is to make a noise made to make fun of a person or persons, made by sticking out the tongue between the lips and blowing to make a sound similar to flatulence.