The DS1307 is a low power Full Binary (BCD) Real Time Clock (RTC) IC with 56 bytes of SVRAM that communicates through I2C Protocol. The IC can work from directly supply on Vcc and switch to Battery automatically when required.
RTC means Real Time Clock. RTC modules are simply TIME and DATE remembering systems which have battery setup which in the absence of external power keeps the module running. This keeps the TIME and DATE up to date.