Hey there! I'm happy to help!
Both South Africa and England have a similar populations. However, these countries have different population densities. Population density is how crowded the country is with people. South Africa is a much larger country in terms of its area while England has a smaller land area. If they have similar populations, but different amounts of space, one is going to have a higher population density. Since England has less space, it has a higher population density as there are more people in a smaller space. There is more land area in South Africa, so this naturally makes it less crowded.
Have a wonderful day and keep on learning! :D