People wore long flowing robes. They would tuck them into their belt to keep the robes out of the way while they worked. In order to show this implicit information, this could be translated as “Let your clothing be kept tucked into your belt in readiness to serve” or “Be dressed and ready to serve.” (See: [[rc://en/ta/man/translate/figs-explicit]])
This is a simile. It compares the disciples being ready for Jesus to return to servants who are ready for their master to return. It is also the beginning of a parable. (See: [[rc://en/ta/man/translate/figs-simile]], [[rc://en/ta/man/translate/figs-parables]])