electrons are held in an atom by net attractive forces .what does it mean?
According to the standard model of particle physics, the fundamental forces (strong, weak, electromagnetic and gravity) are predicted to occur as a result of an exchange between particles via "force carrying particles". Also, neutrons and protons are made up of tinier particles called quarks. And it is the quarks that exchange force carrying particles between each other to give rise to the strong force. The force carrying particles are called gluons.
It should be mentioned that the strong force only operates at EXTREMELY small distances. These distances are on the order of a 1000th millionth millionth of a meter (10 to the power of -15). If you think about a micrometer (one hundredth the size of a human hair), it is a billion times smaller than that.
The strong force also attracts protons to protons or neutrons to neutrons. In the case of protons to protons, the strong force loses strength after the distance mentioned above and succumbs to the electromagnetic force which pushes the protons apart. In this case the force carrier of electromagnetics is the photon (constituent of light).
So in the nucleus there is a delicate balance of the strong force pulling the atoms in to each other and the electromagnetic force which pushes protons apart. It is only when they are so close together does the attractive strong force overpower the electrostatic.