Hi Aditya. A very good question here. Response Spectrum model is our way of predicting structural responses due to earthquake-type and shock-type loads. So, the characteristics of this analysis are: "short", "non-deterministic", and "time-dependent". You may consider this figure.
You are correct, the x-axis here is the natural frequency of the system. To fully understand what the natural frequencies are, please refer to this material https://youtube.com/playlist?list=PLtt6-ZgUFmMLGWlLxHN_TB6MXsWbNoFaG&feature=shared. We will learn this material in 3-4 weeks from now.
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