R is a programming language. It is considered a tool within OHDSI because several R packages exist that were created by OHDSI members.
R functions (vignette link) exist that can translate code in MS SQL dialect to other database dialects. R functions also can assist with generating final SQL that contains “parameters”.
Some analyses also provide R code that executes the analysis and also produces CSV files that can be forwarded to the analysis aggregating investigator.