Both are measured from entry and usually expressed in R or in price distance. A trade that closed at +1R after going as far as −0.7R against you had an MAE of 0.7R. If it reached +2.5R before falling back to +1R, its MFE was 2.5R.
Plotted across many trades, they show how well stops and exits fit the way your trades actually behave. If winners rarely go beyond about 0.5R against you, a stop at 1.5R may be wider than needed. If most of your winners have an MFE far above where you exit, you may be leaving a lot on the table.
They help separate entry quality from exit quality. A low MAE on winners suggests a good entry; a large gap between MFE and the final result suggests an exit problem.
The measures need price data between entry and exit, which a simple trade list does not contain. Many trading platforms and some journaling software calculate them. They are also descriptive: they tell you what happened to past trades, and changing a stop on the basis of them should be tested on enough trades.
Reflect works from the trade result and execution details in your journal and does not calculate MAE or MFE.