ESP32 Dev Board DTR/RTS
EN is enable... Or nrst pin. IO0 is a boot mode pin. When the esp32 chip exits reset, it samples io0 and if it's low it will enter programming mode.

This enables the dev board to reset the board and automatically select the correct values for those pins when programming.
See: https://github.com/espressif/esptool/wiki/ESP32-Boot-Mode-Selection