The abbreviation Fe stands for Ferrous material
i.e., Iron from which the reinforcement steel bars are manufactured.
The number 415 indicates that the Yield Strength (tensile strength when it is subjected to elongation) of steel is 415 N/mm² the bars are designed and manufactured to have.
Similarly Fe250, Fe415 , Fe500 , Fe550, Fe600. The numbers 215, 415, 500, 550 and 600 tell about this Yield Strength in N/mm² the bars are designed and manufactured to have.