In the UK, 'learning difficulties' is the term preferred by the people who have to carry the label. The approved UK government term is 'learning disabilities', but the public still tends to recognise the phrase 'mentally handicapped' - although people with learning difficulties object to it. In the US, the equivalent term is 'mentally retarded' - though to British ears that's highly offensive. The US term of 'developmentally disabled' is sometimes used as an equivalent, though in fact (we understand) it covers other kinds of disability which occur early in life. 'Intellectually impaired' - or 'intellectually disadvantaged' - are other alternatives.
People who, from birth or very soon after, have had a reduced intellectual ability, which has meant that they have not achieved a normal level of the skills required for living.
As with other human characteristics, people vary in their intellectual ability around an average level (the notional though not vey helpful IQ of 100). Many people with intellectual abilities below average manage perfectly well all their lives without special help, and without being labelled as 'handicapped'. In a very real sense they only become handicapped when they get caught up in the machinery of health, education, or social services and get a label stuck onto them.
There are also people - perhaps 3 in every 1000 of the population - who have more severe intellectual disabilities resulting from various causes around the time of birth: Down's Syndrome (or Down Syndrome) is the type of disability best known to the public, though there are many other causes. They may also - though not always - have physical disabilities.
There have always been people with severe learning difficulties, so they too are a 'normal' and 'ordinary' feature of human life and society. In spite of this, people with intellectual disabilities have for hundreds of years been subjected to injustice: removed from ordinary society; denied ordinary opportunities and rights; and even been killed. Western societies may not be as systematically vicious towards disabled people as in times past, but injustice remains commonplace, brutal mistreatment by no means unknown.