I don’t understand the ‘freeze sort order’ explanation, can you make it clear to me, please?

The activities are per default sorted by the column “#”. This means that the header ‘#’ is high lighted blue and that the tasks are listed in their creation order:
If you choose to sort your tasks according to another column (for example by priority, so you click onto the ‘Priority’ header and see it getting high lighted blue)…

… and need to still have ascending values in the  ‘#’ column, simply call “Edit > Freeze Sort Order”.

Merlin will apply your sorting order as the standard order and renumber the ‘#’ values:

Note: When having sorted by a column, Merlin does not allow manual reordering. Use the ‘Freeze Sort Order’  in case you want to move some activities to another position while keeping the current sorting order for the rest.