An informal education is simply learning a trade from someone else.
It’s possible to have a formal education and an informal education.
The formal education is what most employers would prefer to see because it is easier to prove. Having a degree in a specific field will mean that you have certain knowledge that will translate into a better paying job and the company that’s employing you getting a qualified person for the job.
An informal education is what many people end up having. You work under someone who has a degree or has been doing it for enough years to be knowledgeable about the subject. Many car mechanics and other "trade” skills usually have an informal education as to what they’re doing.
Informal education can also be referred to as life experience. After going through life for so many years, you’ll naturally acquire some knowledge about different things that you may not even learn after going through a formal education. This life experience can often be translated into college credit if you are looking to go back to school to either get or finish a degree.
A formal education will ultimately cost more money than an informal education, however the pay for the aforementioned usually makes up for the initial investment required. At the end of the day, some sort of education is better than nothing and will help you to find a good job, if not a great career.