ESPTerm is web terminal emulator on the ESP8266. It starts a WiFi AP by default, and works from mobile browser, so you connect a flashed ESP with 4 wires to openwrt uart and than can manage it from the mobile phone. Due to cheapness you can leave it inside.
As I've badly explained ESP8266 uses is 3.3V for power and ttl lines are 5V tolerant, so you can't power it with 5V without voltage regulator. (But there are ESP8266 modules with 5V->3V regulator on the board.)