What does it mean?
A sub-solar point is when the sun is directly overhead at a particular latitude. When the sun is at the zenith (the highest point in the sky) its rays will be hitting a particular point exactly perpendicular to the surface.
This will make your shadow be exactly under you, making it look like there are no shadows. Take a small jump and you can see your shadow.
How often does it happen?
It occurs twice a year.
To know where exactly it is happening now and when it will happen next you can download the Zero Shadow Day app from the Astronomical Society of India.
