What does this mean?

Yesterday I had a letter from a college that i have applied to do A levels and it says "congratulations! You have been reserved a place on an A levels course" and it has asked me to go to a personal advice session in two weeks

Can anyone tell me what it means please?

Thank you

Also can i change an A level because I don't want to do maths i want to do sociology instead

It means that the college is happy to accept you, and has space for you on the courses you applied for. You can ask them if you would like to change subject, and they'll probably agree, but there may be timetabling issues which means they can't.