What is the meaning of 415 in fe 415?

The abbreviation Fe stands for Ferrous material ie ironfrom which the reinforcement steel bars are manufactured.

The reinforcement steel bars used in RCC (Reinforced Cement Concrete) are designated as Fe415 or Fe500 depending on their Yield Strength. (The numbers 415 and 500 tell about this Yield Strength in N/mm2 the bars are designed and manufactured to have.)

Fe415 indicates that the Yield Strength (tensile strength) of steel is 415 N/mm2.