Proofreading is the name given to the process whereby you are given a section of text and are required to check it for mistakes.
There are all sorts of mistakes that could appear. They might be spelling mistakes, they might be grammatical errors (for instance the wrong text or an apostrophe in the wrong place) or more subtle: for instance the diagram referenced on page 27 may actually be on page 28, and so on and so forth.
Who is proofreading suitable for? Well, if you are an excellent reader and have a good eye for detail, and find yourself picking up errors in daily life, then this could be for you.
You will need good concentration in order to not miss errors, so read each word carefully and check any spelling you are unsure of. People will usually ask for a quote based on an hourly rate or a set amount of material that they want to be checked.
Proofreaders can be generalists and work on any copy, or specific to an area where their knowledge can help ensure accuracy - for instance a particular branch of science or maths where it helps to know the terminology and concepts; but the majority will be generalist work.
If this appeals, then you will need to do the usual things - create a website and then offer to proofread content on a few websites for free in return for a testimonial, this will quickly get you a portfolio and from there you can start charging for your work.
If this is not something you have done before, then it would be worth working for a proofreading qualification, there are many home study courses available for around, or under, the £100 mark and these will help give you the basics - you can also read lots of great information and articles online about proofreading, the sort of errors to look for, and so on.