The concept is fairly straightforward. For any aeroplane you have a take-off distance required, and a landing distance required (TODR and LDR).
Altering paramaters such as profile drag, residual thrust, weight, etc to alter these values is an inevitable practice in design. Generally (not universally) decreasing TODR tends to decrease LDR and vice-versa. What a designer tries to achieve is a balanced field length that is that TODR=LDR for the same aeroplane.
In effect, this means that the field length for all operations should have been optimised to give as short a distance as possible.
Having said that, this is really an Engineer's interpretation, and it may be that the term is used differently in ops.
G