It means that the forces that tend to contract the star (gravity) and the forces that tend to make it bigger (gas pressure, radiation pressure) are in equilibrium, so that the star doesn't get bigger nor smaller.
Pressure caused by fusion reactions in the core is balanced by the gravity exerted by the sun's mass. The balance between pressure caused by heat and gravity caused by the star's mass
Static Equilibrium: If some forces are acting on a body horizontally or vertically, and the body remains it states of rest is called Static Equilibrium. Formula: ∑Fx =0 ∑Fy =0 Example: a book...