What does the idiom "school is the main thing, love is the auxiliary thing" mean?

It's not an idiom to give priority to studies and give priority to love. There is only one idiom including study:

Make academic achievements

Xu Yuqing

Interpretative research: learning. Make some achievements in study

The source is Xiao Gan by Zong Yi, Ming Dow: "Although my wife is overjoyed and doesn't know it, is it also due to her husband's academic success?"

Structural subject-predicate idioms

Used as predicate, attribute and object; Used for study, etc.