Hi Alvin. This is actually a very good question. In order to understand the fundamentals of modal analysis, I will give you a simplified explanation.
The basic idea of modal analysis is to obtain two things: natural frequencies and mode shapes. Natural frequency is a frequency in which one system experiences a phenomenon called resonance. To understand mode shapes, try to think of them as "expected" deformation shapes due to excitation at a certain natural frequency. Hence, the deformation shown in the modal analysis is "fake"!
Now consider this figure.
As the number of modes increases, we can simply say that the accuracy of our model is also increasing. To determine whether we already extracted "enough" modes or not, we can evaluate the ratio of effective mass to total mass (should be close to 1, in this case, 0.88). There are some parameters that will not be explained in this reply.
We will continue to learn this exciting concept in 3-4 weeks from now. However, please feel free to ask and discuss this on the forum/group chat.