"They are but burs, cousin, thrown upon thee in holiday foolery. If we walk not in the trodden paths our very petticoats will catch them." what Celia was trying to explain? and what 'trodden path' means here?

Here Rosalind comes clean about her love for Orlando. Rosalind is all depressed because Orlando seems out of her reach. Celia is trying to cheer her cousin up by stating that her troubles are not really thorns but those sticky "burs" which she can get through. Celia is really trying to lighten Rosalind's mood by convincing Rosalind that her love should be viewed as a romantic adventure rather than a depressing or painful time.

"Thrown upon thee in holiday foolery", please explain this sentence also.

throw the burrs in enjoyment