The documentation says
Radius: Is used to specify the heatmap search radius (or kernel
bandwidth) in meters or map units. The radius specifies the distance
around a point at which the influence of the point will be felt.
Larger values result in greater smoothing, but smaller values may show
finer details and variation in point density.
So if your layer has a projection system in meters, it means the radius of influence around each point will be 8 meters. If it is in feet, the radius of influence will be 8 feet... and if the dataset is not projected but it rather in lat-long, it will be 8 degrees - which is huge.