redeeming (v.16) — the meaning here is “to buy up for one’s self or one’s advantage”
Redeeming the time (v.16) — It literally means, “Buying up the opportunities.” Just as people go out bargain hunting and say, “There, if I buy that today, I can get it at a good price, much better than if I have to let it go until another time. It is worth my while to buy these bargains up at this rate.” Let the Christian be just as eager, just as earnest, to obtain opportunities to witness for Christ, to serve the blessed Lord, and to be a means of blessing to others with whom he comes in contact. Buying up the opportunities, seeking to use them to the glory of our Lord Jesus, realizing that the days are evil and the time for serving Christ is slipping fast away, and that opportunities once lost will never be found again. Therefore, the importance of buying them up while we have the chance. — Ironside, page 163-264.
time (v.16) — not chronos, “time as such,” but kairos, “time as regarded in its strategic, epoch-making, seasonable, opportune seasons.” The idea is not to make best use of time as such, which is what we should do in the sense of not wasting it, but of taking advantage of the opportunities that present themselves. — Wuest, page 126.
evil (v.16) = evil in active opposition to the good, pernicious
do not be (v.17) = stop becoming — forbidding the continuance of an action already going on.
unwise (v.17) = without reason, senseless, foolish, without reflection or intelligence, stupid
understand (v.17) = to set or bring together, to put the perception with the thing perceived, to set or join together in the mind — reflective thinking