。
She might know the truth.
Literal
She [topic-は] truth [object-を] knows [might-かもしれない].
~かもしれない expresses possibility — 'might, may, possibly.' It attaches to the plain form of the predicate and expresses less certainty than ~だろう ('probably') and more openness than ~と思う ('I think'). 真実 (しんじつ, 'truth') is more literary than the everyday 本当 (ほんとう), giving the sentence a slightly dramatic or serious tone.