Foreign language interface (FLI) routines are C programming language functions that provide procedural access to information within Model Technology's HDL simulator, vsim. A user-written application can use these functions to traverse the hierarchy of an HDL design, get information about and set the values of VHDL objects in the design, get information about a simulation, and control (to some extent) a simulation run.
ModelSim's FLI interface is described in detail in the ModelSim FLI Reference. This document is available from the Help menu within ModelSim or in the docs directory of a ModelSim installation.