Isolator

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as the name sugests its to isolate somthing normaly electrical, like a switch to isolate for repairs or faults, google will provide the answer in detail

Well Sunil as Mr.Peter rightly said an isolator is to isolate the faulty part of the section from rest of the section.It is like a mechanical switch used to cut the supply from the faulty part under repair,the diference here is before maintenence starts the substation operator trips the circuit breaker and the thus the supply to the feeder is tripped,but the repair people cannot visualise that as the male female contacts inside the circuit breaker are not visible to them,hence for added safety the isolator switch is opened because the repair staff can physically see the disconnnected portion from the busbar.

The "ISOLATOR" in general it isolates one part to another without any means of physical continuity. In electrical terminology or technical aspects... isolator/s it served as disconnecting switch barely vissible so as to make sure that there is no physical contact to a part/section from where it was. This is normally intended for use for electrical maintenance section by section for personnel safety and not to interupt the entire operation such as in industry(factory), zone power distribution(commercial power). This may either isolate the power feeders, transformers, etc. while under maintenance or system upgrade.

One additional thing

Normally isolators will be used in sub stations as an offload switch.

It will be placed before the breaker (Making sure to the person working in the line after the isolator part, that the system is isolated).

Correct me if iam wrong.