Jupiter is the largest planet in our solar system. But Jupiter is still small compared to the sun. You could fit a thousand Jupiters inside the sun!
Still, Jupiter isn’t massive enough to be a star. Stars have to have enough mass to get hot enough inside to spark thermonuclear fusion reactions. Those sorts of reactions produce energy – and that’s what makes stars shine.
Jupiter would have to have 80 times more mass than it does now, in order to ignite in its interior and shine as stars do. If it had this extra mass, the thermonuclear fusion reactions in its interior would cause it to shine.
Since that’ll never happen, you sometimes hear people called Jupiter a failed star.