Transverters are RF upconverters and downconverters. They allow a radio to transmit and receive on bands outside the range it normally operates on. See the Wikipedia article for more detail.
While these devices are less commonly used with ham equipment (which is designed to directly work on frequencies of interest), they are more common with specialised devices such as those used for Software Defined Radio (SDR), which often have a much more limited frequency range.
|Device||Upconvert/transmit?||Downconvert/receive?||RF (real) band||IF (converted) band|