A prospective meta-analysis uses only trials that have been identified and selected before their results are known. This avoids some of the problems of meta-analyses.
Usually, meta-analyses are performed when individual trials have finished. However, collecting studies after their completion can lead to statistical problems. For example, if a meta-analysis is carried out because of knowledge of a particular positive trial result, this can influence how other studies are chosen for the meta-analysis and cause bias in the analysis. This is why prospective meta-analyses are performed.