is a problem caused by a shadow which blocks the flow of solar energy on the PV module and this decrease the amount of electricity produced by the panel.
When a cell is shaded by a tree branch, building or module dust, the shading on just one cell can reduce power output to zero. The output declines proportionally to the amount of shading. This is significant because every cell in the cell string has to operate at the current set by the shaded cell. This prevents the unshaded cells from operating at maximum power.
For solar systems owners they must first be sure that there are no nearby trees, building or other obstructions like antennas which may cause shading issues.