what is receiver over load

Signal amplifiers are supposed to be linear. That is, the output is a magnified but otherwise unaltered version of the input. But too much signal can make an amplifier non-linear, usually clipping off the tops and bottoms of the sine waves. When this happens, all channels are affected, not just the one that is too strong. In fact, the too strong signal is usually not a TV station. A close FM station or police station is more likely.

If you add a good amplifier to your antenna system and your results get worse instead of better then you have overload, and you need to reconsider more carefully what you are doing.

Overload never causes any equipment damage.

An attenuator is a resistor network that can be used to reduce the gain of an amplifier. 3 dB and 6 dB attenuators are commonly available. If an antenna system needs two amplifiers, where the output of one amp feeds into the other amp, too much gain (overload) can result and an attenuator is usually the simplest solution. If you don’t have two amplifiers, it is unlikely that you will ever need an attenuator.

If you are close to an FM station, there might be a narrow range between too much and too little amplifier gain. (Too little gain = dropouts, too much gain = overload.) You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna. VHF preamplifiers usually include FM traps that can optionally be disabled. Freestanding FM traps are also available. FM traps can either cover the entire FM band or can be single frequency traps that you tune to the offending station. The former are less effective and tend to attenuate channel 6. If the FM station is close enough you might need more than one FM trap.