Effort represents an investment of a fixed resource, like calories.

Paying attention seems to be linked to effort, since deliberate control of attention take effort.

Taking action is hard because it requires effort. Some things feel easy, while others feel difficult. This is a reason we don't always act on our plans.

You can look at effort in the context when you need to make a choice that involves internal conflict. Part of you wants to eat more, while the other part thinks you shouldn't.
Internal conflict requires you to move "uphill" or resist the easy flow.