What is rfid tag collision?

Answer 1

Tag collision occurs when more than one transponder reflects back a signal at the same time, confusing the reader. Different vendors have developed different systems for having the tags respond to the reader one at a time. These involve using algorithms to "singulate" the tags. Since each tag can be read in milliseconds, it appears that all the tags are being read simultaneously.

Answer 2

Tag collision in RFID systems happens when multiple tags are energized by the RFID tag reader simultaneously, and reflect their respective signals back to the reader at the same time. This problem is often seen whenever a large volume of tags must be read together in the same RF field. The reader is unable to differentiate these signals.
To avoid this problem; many systems use an anti-collision protocol (also called a singulation protocol). Anti-collision protocols enable the tags to take turns in transmitting to a reader.