Okay, imagine you have a toy car and a remote control that you use to make it move. Now, imagine you want to play with your toy car near some big machines, like a generator or a radio transmitter.
Mil-std-461 is like a set of rules that grown-ups made to make sure that your toy car and your remote control won't get messed up by the machines. It tells people who build machines how to make sure they don't accidentally interfere with other electronic stuff, like your toy car and remote control.
For example, it might say that the machines can't be too noisy or can't send out too much energy that could mess with your toy car's signals. That way, you can still play with your toy car and remote control, even if there's noisy machines nearby.
So, just like how your mom and dad make rules to keep you safe and happy, Mil-std-461 is a set of rules that grown-ups made to make sure that electronic things can work properly around other electronic things.