A person who sees someone or something "in the flesh" is actually in the physical presence of that person or thing. This phrase is often used in reference to someone that a person may have heard about but is now finally getting a chance to meet. It can also be used when someone has seen someone else only on various forms of media but now actually sees them in person. The phrase gets its meaning from the fact that someone who sees someone else "in the flesh" could conceivably touch their flesh.
There are certain circumstances which may require a type of speaking that is more familiar than formal. When these occasions arise, it is a good idea for speakers to use idioms. Idioms may have accepted meanings that are not quite similar to what one might expect their meanings to be based on the definitions of the words they contain. Such phrases are often recognized by other members of the culture who are familiar with them. One common idiomatic expression is the phrase "in the flesh."
If someone meets someone else or sees something for the first time, it is often an occasion to utter this phrase. There may be times in a person's life when he or she is aware of someone else but hasn't yet had the chance to meet that person. For example, someone might say, "I heard a lot about you, but it's nice to finally meet in the flesh."
This phrase is also useful when someone gets the chance to actually see or meet a famous person for the first time. It is not uncommon for people to get used to seeing someone on television or in the movies without ever seeing them in real life. When that actually does occur, it can be a memorable experience and may warrant the use of this idiom. As an example, consider the sentence, "I've always loved her movies so much, so it was a real thrill to see her in the flesh."
When someone actually sees a person in real life rather than just hearing about them or seeing them on a screen, the difference is that the person may actually be able to touch the flesh of that person. This fact is what gives this phrase "in the flesh" its meaning. It is typical of idioms in that its literal meaning has expanded into the way the phrase is actually used.