Someone is said to be “making a mental note” when they try to remember something which requires special attention, such as the need to pick up a quart of milk on the way home. The term is also sometimes used a bit sarcastically to illustrate a situation, which why you might hear someone say “mental note: do not continue speeding when you see a police car parked at the side of the road.” In the first sense, trying to remember something may not always be effective, as everyone who has forgotten a crucial ingredient at the grocery store, an important birthday, or an anniversary is well aware. The term is also used to make an aside in text, often with embedded sarcastic commentary.
Typically someone makes a mental note when he or she is in haste, and does not have the time to write something down. For example, someone might make a mental note that he or she is out of bagels, with the intent of picking up some more at some point during the day, or someone might see that a store is having a sale and make a mental note to tell a friend. The implication is usually that something is reasonably important, but perhaps not crucial.
The human brain is an extremely complex instrument, and it is notorious for playing tricks with memory. Some people find that they can remember last week's grocery list in perfect detail, but not a password they just made to access an online account. Because of this notorious foible, it is generally idea to turn a mental note into a physical note, to make sure that the thing which requires attention is truly attended to. Carrying a small notebook with reminders will also allow you to jot down phone numbers and other information in a central location, making it easier for you to keep track of these things.
If you have to make a mental note about something, you may want to try and tie it in with something else to ensure that you remember it. Some people literally tie on reminders of mental notes by typing a piece of string or a rubber band around a finger, but you can also set up a mnemonic or phrase to remember something. For example, if your garbage pickup is always on Thursday, perhaps you can think up an alliterative pun using “Thursday” and “trash” to remind yourself to take out the garbage in time.
Many people use the term “mental note” as a bit of flourish, often to point out that something is preposterous or obvious. Rhetorical commentary on various situations may include an incitement to make a mental note which references the salient point of the commentary. For example, a columnist might discuss a story about someone attempting to bribe an agent of the Internal Revenue Service, and the headline of the column might be: “Mental Note: Bribing IRS Agents Does Not Usually Turn Out Well.” In the more preposterous realm of things, a columnist commenting on a story about someone who was randomly persecuted for doing something like taking a snack onto a plane: “Mental note: apparently taking bananas on a plane for a snack is not a good idea.”