To buy additional quantities of something (to supplement what one already has).
牛乳が足りないから、帰りにもう一パック買い足す。
We're short on milk, so I'll buy one more pack on the way home.
パーティー用の飲み物を買い足しておいてください。
Please buy additional drinks for the party.
予備が必要だったので、スペアの電池を買い足した。
I bought extra batteries because spares were needed.