How to secure web api 2 with OAuth?

Question

I went through each and every thread of C-SharpCorner and stackoverflow on this topic but not a single one is well explained, can anyone give me a clear picture of OAuth and Owin in the context of web api security.my questions are:-

1.what is meant by middleware ?

2.what is meant by owin middleware?

3.How to Authorize my client for specific resources with OAuth?

4.Is the purpose of Owin for self hosting in general is different than in OAuth implementation and in OAuth Owin plays different role than decoupling host and web api ?

 

Note:- My questions Is not about .NET CORE.