I know that optical activity means polarisation of light but what is so special in that that it changes the properties of the substances significantly
TL;DR version: Chirality is a basic property of a molecule, while optical activity is a bulk physical property possessed by samples of chiral compounds. Specific rotation is an intrinsic property of chiral compounds and a measure of optical activity under standardized conditions. Optical rotation is not responsible for the disparate behavior of enantiomers of biologically active compounds.
Chirality is an intrinsic property of a molecule (or, indeed, of certain geometric structures, more generally). A chiral molecule is one that is not superposable on its mirror image molecule (its enantiomer), meaning that no series of rotations or translations will make the two geometrically identical in three-dimensional space. One of the consequences of chirality is that chiral molecules in bulk will measurably rotate plane-polarized light by a specific amount and in a specific direction. (The distinction here is crucial, since any molecule[s] will in general cause some rotation, but the random distribution and spatial orientation of the large number of molecules in any given sample will produce no observable net rotation if the molecules are achiral. The rotation caused by one molecule is likely to be negated by another identical molecule in an opposite spatial orientation.) The key point is that chirality is an intrinsic property of a molecule, while optical activity is a bulk physical property of said molecules gathered en masse. Specific rotation, on the other hand, can be viewed as an intrinsic property, since it is defined for a specific sample under an exact set of conditions (light wavelength, path length, concentration [for samples in solution], density [for pure liquid samples], temperature).
Optical activity is not the cause of any changes in the properties of chiral compounds, nor is it responsible for the sometimes wildly different biological effects of certain enantiomers (which I presume may be what you're actually referring to, forgive me if I'm wrong). The enantiomers of chiral molecules can often differ widely in their biological effects due to the basic nature of proteins, which are the constituent biological molecules that most if not all biologically active substances target in one way or another. Proteins are constructed from amino acids, which often themselves contain multiple chiral centers, and these proteins go on to fold into very specific and complex conformations. Enzyme active sites and cell receptors (among various other biological structures) are mostly comprised of proteins with very specific three-dimensional structures. As such, they interact significantly only with molecules that have the appropriate shape. Hence, enantiomers of chiral substances, which differ in their three-dimensional shape, will exhibit variable degrees and types of interactions with proteins, and can hence impact biological targets in disparate ways. This is largely what makes chirality so important to chemists, and partly explains why so much of the most respected (not to mention economically important) cutting-edge work recently (at least in organic chemistry) has revolved around enantioselective or enantiospecific synthesis techniques.
Optical activity is the rotation of the plane of polarized light. It does not change the substances at all; rather, it is a property of the substance itself.
It's special because compounds that rotate the plane of light can either rotate it to the right (+) or to the left (-) and that's measurable! Enantiomers can be differentiated in this way, since they will rotate the light by exactly the same amount but in opposite directions.