This is the 17311th most frequent Russian word.
съесть
To eat (completely).
Here, 'съесть' is used as a verb in its infinitive form, implying a completed action of consuming an object (the apple).
Я собираюсь съесть яблоко.
I am going to eat an apple.
In this sentence, 'съел' is the past tense of 'съесть', indicating a finished action of eating that occurred in the past.
Он быстро съел свой обед.
He quickly ate his lunch.
In this usage, 'съесть' is used as an imperative form, instructing someone to complete the action of eating before a specified time.