Player to be named
A player to be named (aka player to be named later, PTBNL) is a player who is part of a trade but who is not named at the time the deal is made. The player may remain unnamed either because the teams haven't yet agreed on the player or because the teams see some advantage in delaying the completion of the trade.
When the player is not yet agreed on, there are still restrictions on who the choosing team may claim. The teams may either agree on some general class of player who is eligible to be chosen (e.g. any player not currently on the 40 man roster) or a specific list of players. Such restrictions set the approximate value of the PTBNL while still giving the choosing team some extra time to scout the players in question.
There are several reasons why teams may choose to delay the completion of a trade in which the players are known at the time of the deal. One reason is roster restrictions. When a team trades for a player on another team's 40 man roster, they must place him on their own 40 man roster or risk losing him on waivers. If they don't have space on their roster, they may delay receiving the player until they've had time to make another trade that clears roster space for him. Another reason is that players may not be traded while on the disabled list; by listing a PTBNL on a trade instead, the teams can agree to trade the disabled player without falling afoul of this rule.
On rare occasions a traded player has returned to his original team as the PTBNL of his own trade.