In this context, Paul is talking about the importance of love and how it is the most important virtue of all. He argues that even if a person has all the other gifts and abilities, such as eloquence in speech or the ability to perform miracles, but does not have love, then all those other gifts are meaningless.
Paul uses the metaphor of "sounding brass or a tinkling cymbal" to illustrate this point. These objects can make a lot of noise and attract attention, but they are ultimately empty and without substance. In the same way, a person who has all the other gifts but lacks love is ultimately empty and meaningless.
So, the phrase "sounding brass or tinkling cymbal" is used to describe something that is noisy and attention-grabbing but ultimately empty and without substance. It is a warning against being proud of our accomplishments or our abilities, and to instead focus on developing the most important virtue of all: love.