To pay on someone else's behalf temporarily, expecting to be reimbursed later.
昼ご飯を友達に立て替えたので、後で返してもらう予定だ。
I paid for my friend's lunch; they'll pay me back later.
家賃を親に立て替えてもらった。
My parents paid the rent for me temporarily.
会社が出張費を立て替える場合もある。
Sometimes the company advances travel expenses.