Loss of sync is when the TX and RX lose a bit-time somewhere so the bit counters don't agree.
For example, say they send out a 16-bit frame. You get one bit after another and just keep going. Where does one 16-bit word end and the next begin? It's really tough if you lose sync. Sometimes the device generates the clock, and pauses it between words, which is potentially detectable by the controller. Sometimes you can write code to recognize bad words, although just what you can do to figure out what the first bit is can be tricky. If the SPI device has a reset available, by far the easiest solution is to pull it and reset your bit-count to zero.
They sort of depend on never losing sync to begin with, but that's a pretty iffy thing to guarantee.
This is why we created SPI with a Frame or CS wire. You either pull it low to start the frame or it will pull it low to indicate the frame is starting. Microchip's earlier SPI modules don't handle it though. You can handle it on an interrupt basis or you can get a newer part with a hardware Framed SPI Module, unfortunately that hardware module has been a bit "buggy".