SN74LVC1G125: what does 'data retention only' mean as a condition for Vcc range?

"Data retention" means that the device keeps its current state. In other words, if the inputs stay at their old value, the outputs are guaranteed to keep driving the same value. (If V_CC goes below 1.5 V, the outputs might switch randomly, or drive nothing at all.)

Below 1.65 V, the device is not guaranteed to completely operate, i.e., when the value of an input changes, the output might not switch correctly, or not in the specified time.

The "data retention" range extends the "operating" range to lower supply voltages. Depending on how you look at it, the implied maximum is either 1.65 V (because at that voltage, the device begins operating) or 5.5 V (because the device is capable of data retention over the entire range).