Phrase used when advising somebody of facts /an issue /and explanation (etc) using the most simplest of words in order that they may understand.

Often associated with 'full and frank conversation' the phrase 'words of one syllable or less' is actually gramatically incorrect in its use of 'or less' (essentially 'or less' makes no sense whereas 'or fewer' would be correct) but nicely conveys the situation.

Believed to have originated with the staff of Britain's railways in Southern England.

the thing that you don’t have when you’re phones percentage is over 20