The term syncretism refers to a situation where two distinct morphosyntactic categories are expressed in the same way. For instance, in English, first and third person pronouns distinguish singular from plural (I vs. we, he/she/it vs. them), but the second person pronoun (you) doesn’t. Such facts are traditionally understood in a way that English grammar distinguishes between the singular and plural in all persons. However, in the second person, the two distinct meanings are expressed the same, and the form you is understood as a form syncretic between the two different grammatical meanings.
It is important to note that while the two meanings are different, they are also related: both instances of you refer to the addressee. They differ in whether they refer just to the addressee or to a group including the addressee and someone else, as depicted here.
a.you (sg) = addressee
b.you (pl) = addressee + others
The idea that syncretism reflects meaning similarity is what makes its study interesting; a lot of research has been dedicated to figuring out the reasons why two distinct categories are marked the same.
There are a number of approaches to the issue of how relatedness in meaning is to be modeled. An old idea, going back to Sanskrit grammarians, is to arrange the syncretic cells of a paradigm in such a way so that the syncretic cells would always be adjacent. Modern approaches call such arrangements geometric spaces (McCreight & Chvany, 1991) or semantic maps (Haspelmath, 2003), with the goal to depict meaning relatedness as a spatial proximity in a conceptual space. A different idea is pursued in approaches based on decomposition into discrete meaning components called features (Jakobson, 1962).
Both of these approaches acknowledge the existence of two different meanings, which are related. However, there are two additional logical options to the issue of syncretism. First, one may adopt the position that the two paradigm cells correspond to a single abstract meaning, and that what appear to be different meanings/functions arises from the interaction between the abstract meaning and the specific context of use (see, for instance, Kayne, 2008 or Manzini & Savoia, 2011). Second, it could be that there are simply two different meanings expressed by two different markers, which accidentally happen to have the same phonology (like the English two and too). The three approaches are mutually contradictory only for a single phenomenon, but each of them may be correct for a different set of cases.