ARTICULATION

Articulation is to play the notes legato or staccato

When you speak well articulated, everyone can understand what you are saying. When you don't pronounce the words well articulated, no one can understand you.
In music we also use articulation to play understandable en also more beautiful and with feeling. Some melodies sound better when you play all the notes smoothly together, others sound better when you play every note short.

Mostly we use in music a combination of legato and staccato, so a combination of long notes and short notes.
In this case, long notes are ment to play exactly as noted, you play them slured.
Short notes are notes that you have to play shorter than noted.