The Global Horizontal Irradiande (GHI) measures the total amount of light received by a square meter on the ground.   Solar irradiance on a PV panel in the Plane of Array (POA) is a combination of direct solar rays, diffuse irradiance and albedo (irradiation reflected by the ground).
The beam irradiance on a panel or Plane of Array (POA)  is the irradiation coming directly from the sun multiplied by the cosine of the angle of incidence (the angle between the direction of the sun and a vector normal to the panel).

Direct and Diffuse Irradiation on the ground

In addition to direct irradiance, the PV module receives diffuse and reflected irradiance, which are function of the module tilt angle, the soil reflectance factor, and other parameters.

Direct and diffuse irradiation on a tilted module

The following figure illustrates the average annual GHI and POA irradiance for five locations in BC, Canada.  Irradiance on trackers (Orange) is much higher than POA on fixed systems. (Yellow).  GHI (Green) is lower than both .  It shows the importance of setting the panel at an optimum angle or using trackers that optimize the position of the arrays towards the sun.

GHI & POA in various locations in BC Canada