Marginal frequency:
The number in the end column or row - "Total".
Joint frequency:
The ratio of the frequency in a particular category and the total number of data values.
Basically, the values that aren't in the final row or column - "Total".
The number of students who are in 9th grade and play two or more sports is a joint frequency.
This is only asking for the students in 9th grade who play two or more sports.
The number of students who are in the 11th grade is a marginal frequency.
This is asking for the total number of students in 11th grade.
The percentage of the student body that plays one sport is a marginal relative frequency'.
This is asking for the total number of students who play 1 sport.
The percentage of the student body that are in 12th grade and do not play a sport is a joint relative frequency.
This is only asking for the students in 12th grade and don't play sports.