Someone or something specialized has a specific purpose or function. A podiatrist is a specialized doctor. With months of specialized training, a person could learn to walk over hot coals. Good thing that doctor specialized in fixing feet.
The word specialized comes from special. Something special is important, notable, or unique in some way. A specialized hammer is made for one, specific purpose. A specialized teacher has expertise in a particular subject. NASCAR drivers use specialized cars, and assemble-at-home furniture comes with specialized tools. The opposite of a specialized thing is a general thing, which could be used in many circumstances. Something specialized is tailored to one job.