How to interpret the ring resonator?

I saw an example in CST about the ring resonator, but I wonder what is its function?
Filter, oscillator, or antenna?

There are many devices that are called a ring resonator, with different applications. Many of these have appeared at CST meetings, so you would need to be more specific, because, for example:
Sapphire/Rutile dielectric ring resonators at microwave frequencies are used to control oscillators in frequency standards.
Stripline etc. ring resonators can be used together with the Faraday effect to create isolators.
And, yes, ring resonator structures are also used in filters and as antenna components

Hi, Stromer,

Many thanks. Could you give more explanation about "ring resonators can be used together with the Faraday effect to create isolators"?

There are various forms.
One of these uses transmission-line ring resonators that include biased ferrite so that the resonances for opposing directions of propagation are at different frequency. You can consider it as a resonant equivalent of a circulator-based isolator. This is rather special-purpose and doesn't appear ever to have been of much interest to academics - so few (if any) publications
Another uses a dielectric ring resonator coupled to biased ferrite rings - e.g.
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