An architecture decision record is a short document that covers important architecture decisions, the reasons behind it and possible consequences.
Example ADR can look like this:
Title Status What is the status, such as proposed, accepted, rejected, deprecated, superseded, etc.? Context What is the issue that we're seeing that is motivating this decision or change? Decision What is the change that we're proposing and/or doing? Consequences What becomes easier or more difficult to do because of this change?
We can easily track history of decision that we made, reasons behind it and check what was the impact. It’s important especially when we onboard new team members or project grows big.