LD stands for linkage disequilibrium. LD is the correlation between nearby variants such that the alleles at neighboring polymorphisms (observed on the same chromosome) are associated within a population more often than if they were unlinked. LD has been used to identify risk variants in disease without having to directly test the true causative marker. Instead, a marker that is highly correlated to the risk variant can be examined as a proxy for the risk variant.
LD proxy search allows you to search proxy variants for a list of input variants. Users need to specify the population type and the r2.
For more details, please read this excellent paper: VanLiere et al. 2008