Question

what exactly is probe ?

Answer 1

A probe is any "device" or "instrument" that can be used to measure, monitor, or test a system. A ruler would be a probe of length. One could use a molecule as a measure of reaction rates. Light in the form of microscopes can be used to probe cells, or in the case of optical fibres see inside the gastrointestinal tract.

Answer 2

Also, in the context of fluorescent microscopy, a probe is similar to a primer in that it has a complementary region of DNA to bind to a target. A modifier is added to the end of the DNA region so that it will fluoresce when hit with black light.

So basically it has the fluorescent region and the "sticky" region. Can be useful in procedures like Florescent In-Situ Hybridization, which can be used to quickly identify pathogenic bacteria by using specific probes on tissue slides to look for bacteria.