Access specifier is just another term for access modifier (both mean the same thing).
There are so many explanations for this on the web, no need to repeat all this. Except if you have a more specific question about this topic than "explain it all"; then please leave it in a comment.
Here's the official documentation on MSDN:
Access Modifiers (C# Reference)[
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But there are a lot of other websites explaining this topic, potentially better to understand:
c# access modifiers - Google Search[
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