The meaning of high and low level:
Triggered at high level means a forward voltage exists between signal triggering terminal (CH1) and negative supply voltage. It generally connects positive supply voltage and triggering terminal together. When triggering terminal has positive supply voltage or reaches triggering voltage, the relay will pull in.
While triggered at low level means the voltage between the signal triggering terminal (CH1) and negative supply voltage is 0V, or the voltage at the triggering terminal is lower than positive supply voltage. If it lowers to a triggering voltage, the relay will pull in. It generally connects negative supply voltage and triggering terminal together