This is the 220th most frequent Estonian word.
kirjutab
"Kirjutab" means "writes."
In this sentence, 'kirjutad' is the informal second person singular form of 'kirjutama', asking a question about the action of writing.
Kas sina kirjutad talle kirja?
Are you writing a letter to him/her?
Here, 'kirjutab' is used as the third person singular, active voice, present tense of the verb 'kirjutama', meaning 'to write', showing an ongoing action.
Ta kirjutab raamatut.
She/He is writing a book.
This usage of 'kirjutasime' is the first person plural past tense form of 'kirjutama', indicating an action completed in the past.