Hi, Kush.
Yes, improvisation is the art of creating music spontaneously ... without having rehearsed it, or learned the part. Of course, it's only through playing for a long time and keeping track of all the bits and pieces that one can form a mental image of the piece of music and play around with it, following the rules.
Finger style players do the same thing ... they also invent the full part as they go, but again, only because they have played so many parts over so many years that they are not so much inventing as reorganizing all the little bits and pieces they have learned over the years into a new and unique 'part'. This is also done by following the rules of music.
Everything hinges around the chord structure of the piece of music they are playing, even if that itself is being 'invented' ... chords are central to all music, even if you're only hearing those chords in your head as you invent a melody.
PlaneTalk will be very revealing, but don't be discouraged if it takes a while to start improvising ... you really have to get familiar with the way music is structured to be able to tinker with it as you go, so to speak, and that takes time.
Know your keys, by that I mean know the related chords for each key. Don't worry too much about #/b keys, concentrate on guitar friendly keys.
Does that help?
Back to the cricket ... Bangladesh are going down.