Le is a grammatical marker used after a V or VP: V-(LE)-N-(LE), e.g.,
tā chī-le fàn le
‘He ate/has eaten.’
Le marks the actualization of an event/state X in time in the form of [X-le]. To be more specific, it is used to mark the preceding X (V or VP, event or state) as temporally realized. For example, in 我感冒了 wǒ gǎnmào le ‘I have a cold.’, the state of having a cold is actualized in time with the required use of le, which asserts the happening of the state (having a cold), even though it is described as present tense in English.
The difference between [X] and [X-le] is the difference between atemporal vs. tempoally anchored event:
他吃榴莲 tā chī liúlián ‘He eats durian.’ -------habitual/atemporal
他吃了榴莲 tā chī-le liúlián ‘He ate/has eaten durian.------- temporally realized
[chī-le] asserts the actualization of the action of eating durian in time;
without le, it describes a habitual/atemporal situation.