Please help me.

It means that the author advises people to go along with the latest style, but to remain firm in their beliefs, even if that belief is not currently popular. (The expression "like a rock" means immovable. The author has constructed a metaphor based upon the current of a stream or river. "Swim with the current meaning to go along with the latests style, "stand like a rock" meaning to resist changing one's principles just because they are unpopular--the currect of a stream or a river is not powerful enough to move a rock.)

Re: Please help me.


"Swim with the current" is referring to following the group. That would be in reference to dressing in the style of the day etc.

"Stand like a rock" is referring to make up your own mind.
"In matters of principle" is referring to what is right and what is wrong.