If I want to design a system that's wireless, is it possible to create one that runs on batteries and I can turn it on/off to conserve battery life? Or let's say it only turns on when it has to do an action then turns off?
You could use (some form of wireless) to put the remote Arduino into a very low power state so close to off. But the receiver would then be off so you cannot use the wireless to turn it on again.
A "system that's wireless" - if you mean WiFi - uses an ESP (ESP8266 or ESP32) instead of an Arduino. 
Some options are:
You could use an ESP8266 or esp32 and have it go in to deep sleep for a set time, turn on, connect to wifi, check if action is required (e.g. check in to an MQTT server) then go to sleep again.
Or go to sleep until an input pin level changes.
Or you can give it control over it's own power (via relay or mosfet) so that when it is turned on it performs some action then turns it's self off again, this way it can have very low if any current draw when off.
Of course the problem is that when turned off or in sleep mode it can not be monitoring the wifi so any signal to turn back on would need to be local / hard wired.
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