The Microverse is an artificial, microscopic universe that exists inside of the battery inside of Rick's Space Cruiser. It is a universe filled with a species of aliens who have formed a regular, everyday society. When the aliens work, they generate a power source that charges the battery for Rick's space cruiser and occasionally, his phone. Most likely, the planets in this universe have been reduced to the size of the bacterial world.
Rick put a spatially tessellated void inside a modified temporal field until a planet developed intelligent life. This then became the Microverse. Rick gave the aliens in the Microverse another invention, known as the Gooblebox, which is a machine that generates electricity that powers everything in their society. The machine is activated by their work. Unbeknownst to them, the electricity that powers their society, also powers the giant battery they're inside of, which gives Rick's ship infinite power, so that he'll never need to pay money, do work, or run out of battery life every again.
In The Ricks Must Be Crazy, one of the aliens, Zeep Xanflorp, thought that it was too much work to use the Gooblebox and so he created a more efficient source of energy called the "Miniverse Battery", which was the exact same thing as the Microverse battery. Because they had this new source of power, they stopped using the Gooblebox and Rick's space cruiser could no longer function. Rick and Morty had to travel into the Microverse. After a big extravaganza, Rick had explained to Zeep, what had happened, which led to a huge fight. Zeep wanted to kill Rick so that the Microverse would still be able to continue on, without them working for him and Rick wanted to keep the battery working. So, they got into a huge fight, trying to kill each other. Rick eventually beat Zeep in a fistfight, but left him alive and left. When Rick tried to start the car again, it worked. He explained this with the fact that, when he got back to his own universe, "[Zeep] knew one of two things were going to happen. Either he would have to toss a broken battery, or the battery wouldn't be broken anymore." Because Zeep didn't want to have his entire world destroyed, he returned the Microverse population to using Goobleboxes for power. That is how the Microverse currently functions and how Rick's ship currently works.