Question

how horizontal distribution of power often is referred to as a system of 'checks and balances'?

Answer 1

because every organization check power of organization at above or below level

Answer 2

power is shared among different organs of the government such as the legislature executive and judiciary. Let us call thids horizontal distribution power beacuse it allows different governments placed at the same lelvel to exercise different powers. such a separetion ensures that none of the organs can exercise different powers. each organ checks the other. this results in a balance among various institutions. 
in a democracy even though the ministers and government officials exercise powers, they are responsible to the parliament or state assemblies. similarly, altough judjes are appointed by the executive they can chec the functioning of the executive or laws made by the legislature . this arrangement is called the system of cheques and balances.