Question

what does tangent parallel to x axis means? For differentiation.

Answer 1

It means the tangent to the curve is a horizontal line. Get it? The x-axis is a horizontal like so a line parallel to it must also me a horizontal line :) Since you mentioned differentiation, differentiating an equation gives you the gradient at a specific point.

Answer 2

mathematically you may say that if f'(x)=0 at some point same 'a' then the tangent is said to be parallel to x-axis at the point 'a'