An acting governor is a person who does the job of the governor when the governor is not able to do it themselves. It's kind of like when your mommy or daddy is sick and can't take care of you, so your grandma or grandpa comes to take care of you instead.
Sometimes governors have to go away for a little while to do other important things, like go to a meeting or take a vacation. When they do this, they ask someone else to be the acting governor for that short time. That person usually already has an important job in the government, like being a lieutenant governor or a member of the governor's cabinet.
The acting governor does things like sign papers or laws, talk to important people, and make decisions for the state just like the regular governor would. But they know that they are only doing this for a little while and that the regular governor will be back soon. It's kind of like being the captain of a ship while the real captain takes a quick break.