Can somebody explain me what is pentatonic thing? I want to learn Blues in the guitar and it's becoming hard..?

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I've been playing guitar for a couple of years and now I've started to learn Blues a week ago, in the Internet (Youtube). I already know the basic riffs, but when I have started trying to learn the lead Blues solos, they (the people in the videos) only talked about Major and minor pentatonics and I have no idea what the hell is that! Can somebody explain me what's that and

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