MIMO (Multiple Input, Multiple Output) is a technology which allows an antenna or multiple antennas to communicate with multiple devices, which minimizes errors and maximize efficiency. The most commonly used MIMO variant is MU-MIMO, which stands for Multiple User-Multiple Input, Multiple Output. In layman's terms, imagine a classroom with one teacher and many students who need help. Since there is only one teacher, it will take the teacher a while to fully help all the students. But if there are multiple teachers, students will get help much faster and better. The same way, a router with MU-MIMO technology will be able to communicate with multiple devices with increased speed and minimal errors.
MIMO technologies are most commonly found in routers, smartphones, laptops, TVs, etc.