"You are to tell others that what you saw about me is true." The disciples had observed Jesus' life, death and resurrection, and could describe to other people what he did.
"I will give you what my Father promised to give you." God had promised to give the Holy Spirit. The UDB makes this explicit. (See: [[rc://en/ta/man/translate/figs-explicit]])
God's power will cover them in the same way that clothes cover a person. This can be stated in active form. AT: "you receive power" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])