to yield = admit defeat in an argument (in this context)
conviction = being convinced (in this context)
Other words you may find difficult:
propriety = properness/adherence to society's view of the right way to behave
is no merit - today we would say 'hasno merit/value...'
understanding = rational way of thinking
Darcy disapproves of Elizabeth's readiness to pay heed to a friend's desires. Darcy had done what he 'should' do (his duty not to submit to Bingley's wishes- at least, I think that's the context - it's far too long since I re-read the book.)