The gain of the op-amp is very high till certain frequencies.
After a certain value of frequency, as frequency increases, the following two effects occurs.
(a) Gain [ Magnitude] of op-amp decreases.
(b) Phase shift beth imp and op increases.
The decrease in gain is due to capacitive components internally integrated capacitors or stray capacitors) in the equivalent circuit of the op-amp.
The cumulative effect of this capacitance due to characteristics of semiconductor devices and internal construction of ap-amp causes a decrease in gain.
For an op-amp with only one break frequency we will represent all the capacitive effects by a single capacitor fig. This fig (a) represents a high freq $h$ model of an op-amp with single breaks frequency.