In F#, signature file contains information about the public signatures. Signatures can be of a set of program elements, such as types, namespaces, and modules. It can be used to specify the accessibility of these program elements.
For each code in F# you can have signature file. This file has the same name as the code file but with the extension .fsi instead of .fs
There are some rules of signature:
F# Signature file Example
This is code file named signature.fs
This, is signature file named signature.fsi
As you can see this file neither implement code nor has business logic. It just provide information about code file in abstract form.