Understanding the attenuation of a signal
Attenuation in this context refers to a reduction in amplitude of some signal component as it passes through a system (in your case, the system being the highpass filter. Linear filters are designed to pass certain frequency components while stopping (or attenuating) others. As the name implies, a highpass filter is designed to pass high-frequency content while attenuating low-frequency content.

As you noted, the tremor features that you're interested in are high in frequency and small in amplitude. Thus, they may be masked by low-frequency components in the original seismogram. A highpass filter can be used to accentuate the high-frequency tremor waves by attenuating any low-frequency waves that might also be present.