Hi Hasan!
Almost all the life we're currently familiar with relies on liquid water. For this reason, the habitable zone of a star is defined as the orbital region around a star in which liquid water can exist.
The exact range of this region is dependent on a number of factors: the most important factor is the nature of the star in question. For a very bright star, the habitable zone will be farther out, as planets are bathed in intense radiation; for brown dwarves, the habitable zone will be relatively close to the star. We can give a general estimate of the habitable zone by knowing the temperature and nature of a star (ie, if it is on the main sequence).
Of course, not all planets within the habitable zone end up being habitable - they also need an atmosphere and a reasonable albedo.
