Rectification is a way of turning an AC signal (-1 to 1) into a DC signal (+/- 0 to 1).  If you don’t know what rectification does, rather than reading a description of it, just plug a Sine LFO into the input and attach waveform meters to the outputs.

For clarification: Shift N-P/P-N means “Shift negative values to positive values/vice versa.”  This is an unusual type of rectification that you probably wouldn’t see used in electronics.