So, you know how in school you have different classes based on your age and what you're interested in learning, right? Well, back in the olden days there were just two kinds of schools: primary and secondary.
Primary schools were for all the kids up until they were around 11 years old. Then, they would go to secondary school, which was for older kids who were learning more advanced things to prepare them for jobs or college.
But sometimes, some kids had a harder time in school and couldn't keep up with the advanced stuff. So, special schools were created just for them called "secondary modern" schools.
These schools were for kids who maybe struggled with reading or math or needed some extra help to understand things. They would still learn important skills like cooking, typing, and woodworking to help them get good jobs later in life.
So, to sum up: Secondary modern was a type of school for kids who needed extra help with their learning, to give them skills that could help them succeed in life.