Is it worth reading the entire book series or can I skip some?

It really depends on which series you're talking about, because each one works differently. Some have a continuous plot where each book is essentially a continuation of the previous one - in that case, skipping books is a recipe for getting lost. Others, like many science fiction or mystery series, let you read books out of order because each one has its own complete story, even though there are characters and a world that develop across the volumes.

My advice is to look it up in reader communities or forums about that specific series - like Goodreads or even Reddit groups. You'll find people who've read everything and can tell you for sure whether you can skip or not. Some books in a series might even be less interesting, and it's not worth wasting time on one that's universally considered weak, but if it's essential to the plot then you'll have a hard time keeping up later.