The word articulation generally refers to how the pieces of something are joined together; for example, how bones are connected to make a skeleton or syllables are connected to make a word. Articulation depends on what is happening at the beginning and end of each segment, as well as in between the segments.
In music, the segments are the individual notes of a line in the music. This could be the melodic line, the bass (Accompaniment) line, or a part of the harmony. The line might be performed by any musician or group of musicians: a singer, for example, or a bassoonist, a violin section, or a trumpet and saxophone together. In any case, it is a string of notes that follow one after the other and that belong together in the music. The articulation is what happens in between the notes. The attack - the beginning of a note - and the amount of space in between the notes are particularly important.