This is the 2nd most frequent Vietnamese word.
là
"là" means "is/am/are" (to be).
The word 'là' serves as a linking verb connecting the subject 'Tôi' with the noun complement 'học sinh'.
Tôi là học sinh.
I am a student.
The word 'là' is used as a linking verb indicating equality or equivalence between the subject 'Điều mà tôi nói' and the noun complement 'sự thật'.
Điều mà tôi nói là sự thật.
What I said is the truth.
The word 'là' signifies equivalence or describes a defining relationship within the conditional structure of the sentence.