According to a research at Cambridge University, it doesn't matter in what order the letters in a word are, the only important thing is that the first and last letter be at the right place. The rest can be a total mess and you can still read it without problem. This is because the human mind does not read every letter by itself, but the word as a whole.
This message serves to prove how our minds can do amazing things! Impressive things! In the beginning it was hard but now, on this line your mind is reading it automatically without even thinking about it, be proud! Only certain people can read this. Please share if you can read this.
Despite the wording of the first one most definitely not having come from Cambridge University and the second one being wrong, in that most native English speakers can probably read that, it's still an interesting thing to know.
It also perfectly illustrates why it's impossible to proofread your own work.
When we know what a piece of writing says - because we've written it - our mind starts to skip over the words much faster. We develop a sort of error blindness. Even when we've read a piece five or six times, there can still be typos in there that we're missing.
This is why it's important to have other people check your work. A fresh pair of eyes can often spot things we can't.
A beta reader is someone who reads your work and largely looks at the characters and plot. They help with the structural edit by telling you what they liked and didn't like, and whether the story works for them. But remember, everybody reads something different in a story. If one beta reader says something doesn't work, it might not be worth changing your story. If three or four beta readers say the same thing doesn't work, then it's probably good to have a rethink.
A proofreader is someone who checks your story for spelling and grammar. They can help with the line edit and often spot mistakes you miss.
After you've finished your story and performed the first edit yourself (both structural and line), it's worth showing your story to a few other people and getting their feedback. As the writer, the final decision on whether to change something or not is entirely up to you. There's a great quote by author Neil Gaiman:
Remember: when people tell you something's wrong or doesn't work for them, they are almost always right. When they tell you exactly what they think is wrong and how to fix it, they are almost always wrong.
This tends to apply more to beta readers than proofreaders, but, at the end of the day, it's your story and you need to make the final judgement call.