The way s are played and connected to one another: anywhere from gently sounded to heavily attacked; held for the full duration of the note value or clipped short; smoothly connected to the notes before and after, or continuously separated.
There are many technical terms for these differences. For tango we mostly need to be aware of two broad kinds of articulation, the extreme ends of the spectrum: a connected , which often creates a quality, and an accented, clipped short . Generally, the strings and singers use a legato articulation, while the bandoneons play in a marcato one.
A note is a sound or tone having two aspects:
1) The primary, auditory one, is pitch. Each note has a unique pitch, with a sound wave frequency measurable in hertz. In many parts of the world instruments are tuned to A at 440Hz.
2) The secondary, temporal one, is duration, called "time value" or "note value". When written or played each note has a specific duration, how long it lasts relative to the beat.
When pitch and duration are combined we get melodic shape and rhythm. Pitch creates melody and gives it direction, the melodic shape; duration provides the melodic rhythm.
Legato is Italian for tied together, meaning the notes are played in a connected way; there is no separation or space between them. Legato playing is part of the quality of lyricism, that is lyrical music.
Music that is smooth and connected, with a flowing character, often with a broad sweeping melody and gentle accompaniment.
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.