A song that switches back and forth between time signatures is in A. mixed meter.
First you have to know what meter in music is, and that is basically the rhythm, or how many beats are found in one bar, and which notes to use to describe those beats. Mixed meter occurs when the author wants to spice things up a bit, and change something in the meter of his or her song. The song used to have a pretty regular meter, but the author places a different time signature at the beginning of each bar, which will produce a music piece that has an irregular rhythm throughout.
