Only one person; being just a single person (no one else). Used to emphasize solitary or sole person.
彼はたった一人で会社を立ち上げた。
He started the company all by himself (as the only person).
試験に合格したのはクラスでたった一人だった。
Only one person in the class passed the exam.
夜道をたった一人で歩くのは怖い。
It's scary to walk alone on the road at night.
彼女はパーティーでたった一人、親しい友人が来なかった。
At the party she had only one close friend who didn't come. / She was the only one whose close friend didn't come to the party.