This is the 37th most frequent French word.
sera
"Will be" (third person singular future tense of "être").
In this sentence, 'sera' is the third person singular future tense form of the verb 'être' (to be), used to indicate a future occurrence.
Demain sera une belle journée.
Tomorrow will be a beautiful day.
Here, 'sera' is used as part of a future perfect structure, indicating a condition that will be fulfilled before another future action occurs.
Quand elle sera arrivée, nous commencerons le dîner.
When she arrives, we will begin dinner.
This use of 'sera' is in the passive voice, where it signifies that the subject will undergo the action denoted by the verb.