Where a joker may stand
Eight jokers live in the American set. A joker may stand inside a group of three or more identical tiles: a pung, a kong, or a quint. Two 6 Dots and a joker can be a pung of 6 Dot. Four identical tiles and a joker can be a quint. The joker is standing in for the same tile as its neighbors.
A joker cannot complete a pair. It cannot fill a single. Two different tiles plus a joker are not a pung, even when the tiles feel related, such as East and North, or 6 Bam and 6 Dot. Singles and pairs on a card, including the single winds of a NEWS-style line or a single number a line prints alone, are played with natural tiles.
How a joker comes back
Jokers in an exposed group can be redeemed. On your turn, after you have taken a tile and before you discard, you may lay a matching natural tile onto that exposure and take the joker back. Say what you are doing, so the table can see the exchange. You cannot call a discard only to redeem a joker. You cannot claim a joker someone has discarded. A joker used inside your own concealed hand is simply one of your tiles until you expose the group.
Redemption does not forgive a group that was never identical. If the exposure is East, East, and North, there is no single tile that the joker represents, because that exposure was not a legal identical group in the first place.
Two checks before the drills
Can you call a discard solely to redeem a joker? No. You redeem on your own turn with a tile you already hold, after you draw or otherwise take a tile, and before you discard.
Can the Charleston move a joker if everyone agrees? No. Courtesy included. Blind pass included. The practice drills ask you to judge groups, not to memorize a card.