If you’re not familiar, here is the technical definition: smooth and connected; without breaks between the successive tones.
A good lot of my students struggle with this, especially on lead lines or on sustained barred chords. For the most part it is a left hand technique where you need to develop the musical sense and coordination between your ears and your left hand that allows you to let notes ring out for their full value, almost letting them bleed into the next one. It’s almost easier to explain for a piano player because you can use the metaphor of walking, one foot always stays on the ground.
The most common way people mess this one up is by plucking a fretted note and then releasing tension in their left hand as they think about the next note, leaving the string dead for the space of more than half the note’s duration. Honestly the way that I really learned this is to have a teacher that made me stop and start and stop and start the blues scale over and over again until I played every note legato. He did this every time we met until I was cured of the problem. The lessons were stressful, but my tone as a guitarist is great as a result.