The Earth is round and radar waves travel (almost) in straight lines. A radar cannot see anything flying low beyond the horizon. The higher the radar or the target, the further it can see.
In practiceLow-flying aircraft and anti-ship missiles skimming just above the waves therefore stay below a ship radar's horizon for a long time. That is why AEW aircraft (E-3 AWACS, E-7, A-50) lift the radar to some 9 km altitude: from there they see low fliers at hundreds of kilometres.
How far a radar reaches is determined not only by its power (Inverse square law) but also by the curvature of the earth. Because radar waves propagate in almost straight lines (line-of-sight, Electromagnetic waves), a low-flying target disappears below the horizon at a certain distance — however much power you have. This is the radar horizon.
The atmosphere bends the beam slightly back towards the earth (refraction), which puts the radar horizon ~15% further out than the optical horizon. With the standard 4/3 earth model the following approximation applies:
Try it yourself:
This is exactly why attackers fly low: a sea-skimming cruise missile at 5 m or a terrain-following combat aircraft stays below the horizon of a ground radar until very close in, which cuts the defence's reaction time to seconds. Countermeasures: siting the radar high (mast, hill), airborne radar (AWACS looks down from above) or over-the-horizon radar (OTH, HF band), which uses the ionosphere to look precisely beyond the horizon (Frequency bands).