Trusted Language Extensions on Amazon allow you to write PostgreSQL user-defined functions for its cloud database offerings in one of several supported programming languages.
But let's take it from the beginning. PostgreSQL allows user defined functions to be written in other languages besides SQL and C. These other languages are generically called procedural languages (PLs). Procedural languages aren't built into the PostgreSQL server; they are offered by loadable modules. That way you can extend your database with powerful features not found in SQL. For instance you can write a PL/Perl procedure to accept a string from your SQL to apply regular expressions to it in order to tokenize it.