Mental exhaustion

What does that mean? Does that mean your brain is working harder or being worked? Does that mean you are a decision maker instead of an individual contributor? I am asking because that's how I feel today. Got some things accomplished and some just piling up on the to-do list. First day back in the office and have 3 drop-ins that took a good 1 hr of my time - all good but still 1 more hr I did not have.