Question

what are localised and non localised fringes?

Answer 1

Non localised fringes occur when you use a point-source.
There can be fringes anywhere in the volume where the two beams out of the interferometer intersect.
Non localised fringes are usually very dim.

If the source is wide, then the different interference patterns produced by the different points in the light source will blur each other. Still, there might be a place where the interference patterns coincide.
In that case, we say the fringes are localised and the fringe pattern is usually brighter since the source is wider.

Examples:
Non-localised fringes can be produced by Young's double hole experiment.
Localised fringes can be obtained on soap bubbles or by using a Michelson interferometer.