What do we mean by "playing outside"? Why would you want to do it? Basically, we're talking about notes that don't belong in the current key or tonality, creating tension and dissonance. That could be just a flat 5th added to the minor pentatonic in a blues solo, but that's a very tiny version of "outside". In practice, we're really talking about a more sustained tension, across a whole phrase. And why do it? It's fun! The word "dissonance" can have negative connotations, but if we frame it as "tension", it's an essential part of music. If we played music with no tension (harmonic, melodic, rhythmic or textural) it would be very bland and dull. The important thing is to recognise the need to resolve, to come back "inside". If you go outside and stay outside, your music will be equally dull, but also very hard work!