Rashi: It means to testify something that is not correct, that completely removes the witnesses from the testimony ¹ - because other witnesses testify that the witnesses were with them in another location and cannot possibly have seen what they claim to have seen. ²

Yerushalmi Makos, 1:7: It refers to a second pair of witnesses, who testify that they were with them in another location. ³

Makos, 5a and Yerushalmi Makos, 1:7: It implies that the second pair of witnesses are not coming to contradict the testimony of the first pair, but on the witnesses themselves. ¹