A Transaction is any movement of money (in or out) in relation to a specific Investment.
For example, the renovation cost of your house which you decided to do, but which was not included in the money you paid to buy the house. Or that extra cash you put into a mutual fund that you had for a while because it seemed to be performing well. All these cash movements may or may not impact the value of your Investments, and will impact one measure of your return, but not others.
A Transaction can be one of four types: