Clutter folding is a term used in describing "clutter" seen by radar systems. Clutter folding becomes a problem when the range extent of the clutter (seen by the radar) exceeds the pulse repetition frequency interval of the radar, and it no longer provides adequate clutter suppression, and the clutter "folds" back in range. The solution to this problem is usually to add fill pulses to each coherent dwell of the radar, increasing the range over which clutter suppression is applied by the system.
The tradeoff for doing this is that adding fill pulses will degrade the performance, due to wasted transmitter power and a longer dwell time.
References
Clutter folding Wikipedia(Text) CC BY-SA