This is another of those areas where the expression used changes every so often, usually when the rabble cottons on to it and starts using it as a term of abuse.
As it implies, it refers to someone who finds it difficult to learn. That does not mean, as you heard, that they cannot hold their own in a conversation, or look after themselves or earn a living. They would probably find a problem with complicated academic concepts though.
There have always been words for the less intellectual of us, some of them unkind. Some of the kinder terms have become used less kindly over the years.
Take the word "silly". In Old English "saelig" meant holy. God was looking out for those who were not good at looking out for themselves, so they were "holy". Even Wordsworth has a line about how the clergy are not doing their job for their flock and "the silly sheep look on and are not fed" Gradually it became an insult.
In the 19th and early 20th centuries, learned physicians tried to classify levels of intelligence, and reasons for lack of it, with terms like "moron", and "cretin" which had specific scientific meanings, but deteriorated into insults.
It can be hard work keeping up with the latest way of not offending people.