It allows the identifiers in the imported package to be referred to in the local file block without a qualifier.

If an explicit period (.) appears instead of a name, all the package’s exported identifiers will be declared in the current file’s file block and can be accessed without a qualifier.

Assume we have compiled a package containing the package clause package math, which exports function Sin, and installed the compiled package in the file identified by “lib/math”. This table illustrates how Sin may be accessed in files that import the package after the various types of import declaration.

Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import M "lib/math"         M.Sin
import . "lib/math"         Sin

Ref: http://golang.org/doc/go_spec.html#Import_declarations