From Powershell Scripts :
According to the variable syntax, if the path to the file is enclosed in curly braces preceded by a dollar sign (as in $ {C: \ foo.txt}), it refers to the contents of the file. If it is used as the value of L, all records associated with it will be written to the file. When used as an R-value, it will be read from the file. If an object is assigned, it will be serialized before it is saved.