To understand the concept of interference and conditions for sustained interference, let us first discuss the superposition principle.

Superposition Principle:

Superposition principle enables us to find the resultant of two or more wave motions.
According to this principle, when two or more waves travelling through a medium superimpose upon one another, a new wave is formed in which resultant displacement . at any instant is equal to the vector sum of the displacements due to individual
waves . at that instant
For example, when crest (or trough) of one wave falls on crest (or trough) of the other, the amplitude of the resultant wave is sum of the amplitudes of two waves.
When crest of one wave falls on trough of the other, the amplitude of resultant wave is difference of the amplitude of the two waves.

INTERFERENCE OF LIGHT:
When the two light waves of the same frequency-and haying a constant phase diffeerece traverse simultaneosuly in the same region of a medium and cross each-other, then there is a modification, in the intensity of light in. the region of superposition. this modification or change of intensity of light resulting from the superposition of two or more waves of light is called interference.
At some points, where the crests of one wave falls on the crest of the other resultant amplitude is maximum. Hence the intensity of light is maximum. At certain othr points, crest of one wave falls on trough of the other. Resultant amplitude becomes minimum and hence intensity of light is minimum. So in other words, this kind of modification in energy distribution is called interference.
Thus, interference of light is also denned as the phenomenon of redistribution of light energy in a medium on account of superposition of light waves from two coherent
source?.
At the points, where the resultant intensity of light is maximum, interference is said to be construtive. At the points where the resultant Intensity of light.is minimum the interference is said to be destructive.

6 CONDITIONS FOR SUSTAINED INTERFERENCE:
To achieve sustained or well-defined and observable interference pattern following conditions must be fulfilled :
(i) The two interfering sources must be coherent i.e. the sources should emit light of the same frequency and their phase difference should remain constant If this condition is not satisfied the phase difference between the interfering waves will vary continuously. As a result the resultant intensity at any point will vary with time being alternately maximum and minimum.
(ii). The interfering waves should have equal amplitude otherwise the minimum intensity will not be zero and there will be general illumination.
(iii) The two interfering waves must be propagated along the same line otherwise their vibrations will not be along a common line.
(iv) The separation between the two sources must be as small as possible. If this condition is not satisfied the fringes of maximum and minimum intensity will . lie so close together that the fringes will not be separately visible.
(v) The two sources must be narrow. This is due to the reason that a broad source is equivalent to a large number of the fine sources lying side by side. Bach pair of the sources will gives its own interference pattern. The fringes of different interference pattern will overlap resulting in a general illumination.
(vi) If the interfering waves are polarised, they must be in the same state of polarisation.

This is all about interference and 6 conditions for sustained interference.