This is the 4993rd most frequent Russian word.
легкий
"Легкий" means "light" or "easy" in English.
Here, 'лёгкая' describes the physical quality of the box being light in weight.
Эта коробка очень лёгкая.
This box is very light.
In this case, 'лёгкий' refers to a personality trait, indicating that the person was easy-going or not demanding.
У него был лёгкий характер.
He had an easy-going character.
Here, 'лёгким' describes the difficulty level of the task, meaning it was not challenging.