"That special someone," most commonly refers to someone that has done something meaningful for you/another individual. The phrase could also
point out to someone who by which has special skills and/or abilities for a certain manner, or even use it in an anonymous manner to a group of people for that only the person it was directed to will know who they meant. Sometimes it can be used
sarcastically for someone who by which is the, "
oddball," or literally the, "special one," out of a group or in any other circumstance.