what are those little chords at the top of each bar in a blues? I was wondering, if I got like a D7, that`s D, #F, A and C, does that mean I can play #F in that bar when I`m improvizing, although #F it`s not part of the scale I`m improvizing in?

any explanations will be more than welcome thanks ahead.

Not sure I know what you mean. But D F# A and C are the notes that make up a D7 chord. So you can certainly use those notes to improvise.

it means what the chord the piano and the bass is playing. each chords have scales you could improvise with