A newbie trap is an informal game term. A newbie trap is a strategy, option, or card that appears attractive (particularly to new players) but is actually terrible. A hypothetical example might be a card that says, "If your opponent plays a character named Roger, Jim, or John, they lose the game immediately." Winning instantly is a powerful effect, but a new player might not recognize that LOTR TCG doesn't have any characters named Roger, Jim, or John. A strategy that is deceptively weak, deceptively complicated, easy to screw up, or just infamously unsuccessful could be described as a newbie trap.
Moria Tentacles and Gondor Wraiths are somewhat infamous for being newbie traps, although they have occasionally been used successfully.
Newbie traps are an example of a negative play experience. Fixing newbie traps is one of the main goals of the PC Errata project.