Notes played between the primary s or emphasizing usually weak beats, such as 2 and 4; in tango almost always as part of a rhythmic pattern. Often the syncopated notes are emphasized or accented in a style, but not necessarily so.
There are three common syncopated rhythmic patterns in tango, the habanera and two derivatives of it. There is great variety in the way the patterns are used. Often just part of them, the most syncopated bit, will be played. That’s true mainly for the first two; the 3-3-2 will be played as notated. Here’s what they look and sound like.
Habanera Variations describes the syncopation and relationship between these patterns and some others. And there are some audio examples from the tango repertoire.
Beat is the underlying and regularly spaced pulse of the music, measured in beats per minute. There are a fixed number of beats in a bar, indicated by the time signature. Tango (2/4, 4/8, 4/4) has 2 or 4 beats per bar, vals (3/4) has 3 and milonga (2/4) has 2.
(There may be a sense of 4 beats even though the time signature is 2/4. Tango very often subdivides the 2/4 beat, doubling the count from 2 to 4, effectively using a 4/8 time signature. Some tango music is explicitly written in 4/8, most are in 2/4. See Tango Time Signatures and the Beat).
Marcato is Italian for marked, meaning the notes are to be accented and emphasized. In tango the notes are also played clipped or cut shorter than the note value as written. That is called staccato. The performance style, the articulation, combines marcato and staccato. And that gives the music a crisp and bold character. When I use the term marcato those are the qualities I mean.
Tango uses marcato style playing very often, especially in the accompanying instruments, frequently the bandoneons but others as well.