Improvisation is an important aspect of Music in general. Musical improvisers often understand the idiom of one or more musical styles - e.g. blues, rock, folk, jazz - and work within the idiom to express ideas with creativity and originality. Improvisation can take place as a solo performance, or interdependently in ensemble with other players. When done well, it often elicits gratifying emotional responses from the audience.
Many neophyte guitarists can get stuck playing the same pentatonic scales when called upon to solo. In music, a pentatonic scale is a scale with five notes per octave. This is a problem that arises when people learn to only use scales in terms of leads and improvising. Solos can be thought of as songs within songs - with their own melodies and dynamic tension. A couple of articles on Guitar Noise that deal with scales and improvisation are Leading Questions and Picture In Dorian Gray.
Listen to how other instruments solo. Trying to imitate phrasings other than the guitar can also inspire you to new things.
When looking for ideas it can be quite useful to look outside of your normal style for a while. Gilbert Isbin has written an article on Improvisation for the Fingerstyle guitarist that shows how different chord voicings can lead to interesting improv ideas.