Regioselectivity refers to the preference of a chemical reaction to proceed in ONE direction as opposed to another. Consider a simple reaction...the hydrohalogenation of propylene by HX...for which TWO products are possible, and are observed...
H3CCH=CH2+HX→H3CCHXCH3+H3CCH2CH2X
MORE of the secondary alkyl halide is obtained....
This major product is the so-called Markownikow product, and the reaction proceeds in this direction of the basis of the enhanced stability of 2∘ carbocation...H3C−+CH−CH3 versus the primary carbocation...H3C−CH2−+CH2
If there is a specific issue or observation, please refine the question... As for any physical or chemical phenomenon, you do the experiment FIRST, and then try to interpret it.
