Reflecting impedance in a transformer
Its the 'equivalent load' you would have to connect across the primary voltage  in order to produce the same value of current that would flow in the primary of a transformer due to a load connected  to the secondary.

The basic theoretical concepts are:

(1) Transformer turns ratios
(2) Power conservation (power out = power in) 
(3) Ohm's law

 

An example:

Suppose you had a 10:1 turns reduction transformer and put 100V on the primary. The output voltage would be 1/10 of 100 i.e. 10 V. If you were to connect a 10 ohm load across the secondary this would produce a secondary current of 1 Amp. (Ohm's law). Assuming the transformer is 100% efficient then the power out must equal the power in. This means the primary current will be 1/10th the secondary current or 0.1 amps. So at the primary side we have 100V and a current of 0.1 amps. 

If the transformer circuit was replaced by a single equivalent load then by Ohm's law this would be equal to 100V / 0.1A = 1000 ohms. This is the reflected impedance of the 10 ohm load on the secondary.