Okay, so MIL-STD-188 is a set of rules that grown-ups use to make sure they can talk to each other using special machines called radios. You know how you use your walkie-talkie to talk to your friends when you're playing outside? Well, these radios are just like your walkie-talkie, but they're much bigger and they're used by soldiers and people who work for the government.
MIL-STD-188 has lots of different rules that help make sure the radios work properly. It tells people how to make sure the radio waves they send out don't get mixed up with other radio waves in the air. It also tells them how to send secret messages that only certain people can understand.
The grown-ups who use these radios need to make sure they follow MIL-STD-188 so they can talk to each other during important situations like wars or disasters. It's kind of like how you have rules you have to follow in school to make sure everyone gets along and learns properly. So basically, MIL-STD-188 is just a set of rules to make sure grown-ups can talk to each other using radios when they really need to.