VI conversion (sometimes called EI conversion) is the name for a process used to make power supplies efficient.
The problem is that in energy terms, a fixed amount of energy at a high voltage means a relatively low current; when converted to low voltage, if the conversion is to be efficient (i.e. low energy loss), there must be a higher current. Similarly, a fixed amount of energy at a low voltage means a relatively high current; when converted to high voltage, if done efficiently, there must be a lower current.
However, if a power supply takes in a given current (at the supply voltage), and wishes to produce a lower output voltage, then unless it has some way to produce more than that much output current. the voltage difference between the two is lost energy, lowering the efficiency. (Linear power spplies usually have this issue.) Increasing the current is not trivial, since without some special mechanism, the number of electrons in the current is fixed, and 'electrons out' must equal 'electrons in'.
An equivalent, but inverse problem is encountered when the output voltage needs to be higher than the input.
There are a number of different design approaches which can achieve VI conversion in an efficient manner.
Switching power supplies usually have some mechanism to do VI conversion, to allow them to operate with high efficiency.