Great to hear that you’re considering studying in the US.
US colleges typically require you to submit transcripts (grades) from your final 4 years of school before high school graduation - this usually corresponds to Grades 9-12 in South Africa.
The midyear and secondary school report are terms used by the Common Application (used to apply to many, but not all colleges in the US) to denote transcripts that should be submitted by your school’s guidance/careers counselor.
Typically, the secondary school report is submitted whenever you apply to your first school - this can be around Nov 1 if it’s EA/ED or could even be as late as Jan 1 if you do not apply EA/ED. The secondary school report contains all of your grades up to that point.
The midyear report is submitted a the midpoint of your final year of high school. However, since you are in South Africa, we are assuming that you are on the Southern Hemisphere schedule. Typically, the midyear report is submitted in January/February AFTER your apply to colleges to update them on your performance. For you, your midyear report will be submitted after your have graduated from high school and will most likely contain all of your final grades.
There is one final submission called the Final Report, but for you this will essentially be the same as your midyear report!
Hope this answers your question!