The "Lock duration" function is designed to lock the step duration (this function is available by a right click on an empty step only).
Example : if you lock the duration at 30 secondes, this duration will stay the same for the duration calculation of the playlist. Event by putting a sound longer or shorter than 30 secondes.
For exemple, this feature can be used for a news bed during the news headlines. Usually the headlines are 20 secondes long but you can put a bed of 1 minute if you need more time when headlines are going to be longer due to important news.