Retrocausality (or retro-causality) is a very strange idea to our conventional way of thinking. Normally we think that if event A causes event B, event B must occur after event A: causality only works forward in time; the present can only influence the future, not the past. Retro-causality is the idea that the present influences or can influence the past: event B can cause, or at least influence, an earlier event A. It is time-reversed causality: causality working backwards, not forwards, in time.

In the article whose title you have quoted (in which "the place effect" should read "the placebo effect"), the authors define "retrocausal" in their abstract:

"This experiment tested whether some placebo effects may also involve retrocausal, or time-reversed, influences."
https://www.ncbi.nlm.nih.gov/pubmed/17931066

The investigators claim to have found evidence of a statistically significant relationship between randomised events and brain patterns before that event. The idea, then, is that that relationship is retro-causal, because the event is influencing what happened before it.

Here is a relatively comprehensible explanation of retro-causality:
https://www.scienceandnonduality.com/retrocausality-can-the-...