Dyslexia can be described as a condition that affects one's capability to manipulate symbols and sounds. It typically shows up as difficulties in reading, going backwards and forwards from letters to words and sounds, to meaning. As people do every time they read aloud, as an example.
A dyslexic person's eyes see things the same as a non-dyslexic's eyes. But with the dyslexic, the brain interprets the data received in different ways. You don't "catch" dyslexia, you are born with it. Approximately 1 out of every 10 has some form of dyslexia, to some degree. Going for a test for dyslexia would be the only way to know for certain whether a person is dyslexic.
A dyslexic person can learn how to do practically anything the non-dyslexics do, but dyslexics learn in different ways. They have to be taught in the manner in which they are able to learn. If not, they might never "get it" by themselves, then become discouraged and stop trying, thereby shutting out a whole sector of learning and possibilities essential for their future success.
Today, school-age children are routinely screened for dyslexia, but it wasn't always that way. In fact, it's only been within the last 15 or so years that screening and testing for dyslexia has become the rule, not the exception.
Just about all adults who graduated from elementary school over fifteen years ago have never been tested. This means that there's around 2 million dyslexic adults in the USA alone.
What normally makes them very difficult to locate and help was the manner the educational system treated them as children. These were not understood. They got branded as dull, lazy, underachievers and mental defectives (which most were definitely not!) They were hurt and humiliated by their differences. As defense mechanisms to shield themselves, they learned to hide these differences.
Today you will find them as people working at jobs way beneath what their intelligence would indicate they were qualified for. This to be able to avoid paperwork, needing to read anything for their work. A straightforward dyslexic test could very well set them on the road to overcoming dyslexia and opening up an entire new world of opportunity...