From White House transcript of what he said at Press Briefing:

ARMED FORCES READY RESERVE PRESIDENT TRUMP: “I also signed an executive order giving the Department of Defense and Department of Homeland Security the authority to activate the ready reserve components of the armed forces.”

PRESIDENT TRUMP: “This will allow us to mobilize medical disaster emergency response personnel to help wage our battle against the virus by activating thousands of experienced service members, including retirees.”

Here’s what he’s actually doing in writing;

Section 1. Emergency Authority. To provide additional authority to the Secretaries of Defense and Homeland Security to respond to the national emergency declared by Proclamation 9994, the authorities under section 12302 of title 10, United States Code, and sections 2127, 2308, 2314, and 3735 of title 14, United States Code, are invoked and made available, according to their terms, to the Secretaries of Defense and Homeland Security. The Secretaries of the Army, Navy, and Air Force, at the direction of the Secretary of Defense, and the Secretary of Homeland Security with respect to the Coast Guard when it is not operating as a service in the Navy, are authorized to order to active duty not to exceed 24 consecutive months, such units, and individual members of the Ready Reserve under the jurisdiction of the Secretary concerned, not to exceed 1,000,000 members on active duty at any one time, as the Secretary of Defense and, with respect to the Coast Guard when it is not operating as a service in the Navy, the Secretary of Homeland Security consider necessary. The Secretary of Defense or the Secretary of Homeland Security, as applicable, will ensure appropriate consultation is undertaken with relevant state officials with respect to the utilization of National Guard Reserve Component units activated under this authority.