My book says: the thermocouple is based on the fact that any two metals in contact generate a tiny voltage, in order to measure this voltage the metals need to form a circuit which means that there must be two junctions. If the junctions are at the same temperature there will be no voltage because the two voltage will cancel out.
So my question is: what is meant by junctions? and why is the voltage canceled out if the temperatures of the two junctions are same?
Thanks in advance

There is no simple answer to your question. Three scientists, Seebeck, Peltier and Thomson, made this discovery. It is call electromotive force (EMF) at the junction formed between two dissimilar metals. When heated at one end a small current is generated. When there is no heat there is no current, cancelled out.