A simple sentence isn’t necessarily as simple as it sounds – it doesn’t have to be short, nor does it have to be easy to understand. A simple sentence is an independent clause on its own.
Those far greater than me have put it pretty well:
‘With regard to simple sentences, it ought to be observed first, that there are degrees in simplicity. “God made man”, is a very simple sentence. “On the sixth day God made man of the dust of the earth after his own image,”’ is still a simple sentence in the sense of rhetoricians and critics, as it hath but one verb, but less simple than the former, on account of the circumstances specified.’(George Campbell, The Philosophy of Rhetoric, 1776)
Notice that there is quite a lot of ‘other stuff’ around Campbell’s second simple sentence, but the core idea – the main thought (in bold) – is still the only part with a verb in it. It is that core that makes the sentence simple. If there is one subject and one predicate (the verb and information following the verb) then you have a simple sentence.
‘A sentence is classified simple even when it has a compound subject or predicate (or both) and includes modifying words and phrases:
You and your friends can see the mountain on your next trip.
You can see the mountain and climb to the top.’(R. DiYanni and P. C. Hoy II, Scribner Handbook for Writers. Allyn and Bacon, 2001)
If you strip away all the rest of the sentence and can still find just one main idea, then it is a simple sentence.