"To label the box" means to write or attach a sticker with information on it to the box. This helps to identify the contents of the box or provide instructions for handling or storing it. Full definition
Essentially, tell everyone to label boxes with exactly what's inside, as well as the room at the new home where the box should be placed. (rismedia.com)
Each group's materials were stored in labeled boxes which remained against the back wall. (edutopia.org)
Be sure to label all boxes clearly with the rooms they are going to. (styleathome.com)