It means to gradually learn more about someone or to become familiar with someone. When you first meet someone, you don't know anything about them. The more time you spend with them, the more you learn about them. That is referred to as "getting to know someone".
If someone is "hard to get to know", it means that it is difficult to become more familiar with them or to learn much about them. They might be a very private person who does not like to reveal many personal details.
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