What is RADAR horizon?

Radar horizons:

The distance of the radar horizon under average atmospheric conditions over the sea is little more than one third greater than that of the optical horizon. It will of course vary with the height of the aerial, and be affected by abnormal refraction.
No echoes will be received from a coastline beyond and below the radar horizon, but they may be received from more distant high ground: this may give a misleading impression of the range of the nearest land.