MU-MIMO stands for Multi-User Multiple-Input Multiple-Output. It's a Wi-Fi feature that allows a router to communicate with several devices at once, rather than switching between them one by one.
In older Wi-Fi standards, even if a router had multiple antennas, it could only send data to a single device at a time. The router would give each device a tiny slot to receive information, and that slot was wasted if the device wasn't ready. MU-MIMO uses the router's multiple antennas to handle several conversations in parallel, sort of like a receptionist managing multiple phone lines instead of putting everyone on hold.
This matters most in busy households. Imagine you're video calling on your laptop, someone else is streaming a film on a tablet, and a smart speaker is playing music, all at the same time. Without MU-MIMO, the router juggles these requests inefficiently. With it, the router allocates antenna resources to serve all three devices together, cutting waiting time and reducing congestion.
MU-MIMO arrived in Wi-Fi 5 (802.11ac) routers and continued into Wi-Fi 6 (802.11ax). Not all Wi-Fi 5 routers support it, so check the specifications before buying. Your devices also need to support MU-MIMO to benefit: most modern phones, tablets, and laptops do, but older ones may not.
When shopping for a router, look for MU-MIMO if you have a multi-person household with lots of wireless gadgets. It won't speed up a single device's connection, but it will make your whole network feel more responsive when everyone is online at once.