What is the meaning of 500Wp in case of solar panels?

In case of a solar panel, 500Wp means 500 Watts peak (Wp), which is the maximum power it can produce under ideal standard test conditions. Typical ideal standard test conditions would mean solar irradiance or the intensity of sunlight of 1000W per square meter, photovoltaic cell temperature of 25°C and an air mass (AM) of 1.5g. The term 1.5g refers to AM1.5G solar spectrum, which is a standard for testing terrestrial solar cells.

1.5 indicates that the light has travelled through a path length of about 1.5 times the possible shortest distance from Sun to Earth. G stands for global, meaning that it includes both direct and diffuse or scattered light. AM0 will be the corresponding value in space beyond the atmosphere. AM1.5 corresponds to sunlight received at an angle of 48.2° from the zenith. It represents the yearly average irradiance from the Sun in temperate (mid) latitudes, where many large population centers are located.

Though that is the peak power output at standard test conditions, actual performance can vary significantly. Temperature may be much higher than 25°C, which reduces the efficiency. Rooftop solar panels in this region with hot climate are usually kept elevated above the roof to give good ventilation below the panels. That will reduce the rise in temperature of the solar panels while in bright sunlight.

Shading from clouds, trees and buildings can decrease the power output of solar panels. Tilt of the solar panel relative to the direction of the sun will also affect the power generated. Even in a specific location, this will vary from season to season and throughout the day. Dirt and debris accumulated on the surface of solar panels can block solar irradiation and reduce power production, mandating periodic cleaning.

Sun tracking solar panels can improve the energy production. It has been in use by some of my amateur radio friends including VU2MIE in Wayanad. One ham from overseas posted seeing a whole solar field with Sun tracking systems in Germany. VU2XPZ mentioned that engineering student projects with couple of light dependent resistors have been used for Sun tracking solar panel systems. Higher grade of control is possible with microcontrollers like Arduino as well.