How do my TMA marks contribute to my overall subject score, considering a 20% weightage for Theory?

This means that the maximum marks allocated for the TMA component of each subject are 20% of the total maximum marks for that subject.
Given the "TMA Marks" shown (e.g., 18, 15), it is highly probable that the maximum possible TMA mark is 20.

If the maximum TMA marks are 20, and this represents 20% of the Maximum Theory marks ($M_{Theory}$), then:

To find $M_{Theory}$, divide 20 by 0.20:

This indicates that the Maximum Theory Marks (which refers to the total maximum marks for the subject) for each subject are 100.

For each subject, the total maximum marks are 100. Out of these, 20 marks are allocated for the TMA, and the remaining 80 marks are for the written theory examination. The "TMA Marks" column in your table shows the marks you have obtained out of these 20 allocated TMA marks.
For example, for Hindi, you scored 18 out of the 20 TMA marks. These 18 marks will be added to your score from the written theory examination (out of 80) to determine your final total marks for the subject (out of 100).