From Wikipedia:
A plenary power or plenary authority is a complete and absolute power to take action on a particular issue, with no limitations.
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In United States constitutional law, plenary power is a power that has been granted to a body or person in absolute terms, with no review of or limitations upon the exercise of that power. The assignment of a plenary power to one body divests all other bodies from the right to exercise that power, where not otherwise entitled. Plenary powers are not subject to judicial review in a particular instance or in general.
It is relevant right now because the MAGA movement wants to consolidate power in the executive branch. Basically, make the president a de facto dictator who is unchecked by Congress or the courts. It is a major part of the Project 2025 document, too.
The scary thing is, they might just pull it off. Over many years Congress has gradually delegated much of its constitutional power and authority to the executive branch. The right realizes they have an opportunity now to grab the rest of it. It is not clear to me how, or if, they will be stopped.