This poem is built around an extended metaphor which, as you can probably guess from the title, compares a mouse to time and develops this description to build up a fascinating picture of time and its effects and impact on man. The key characteristic of time that is developed is the way that time "nibbles" at everything, which of course is a word that means to take very small bites off of something very gradually. The way in which time is constantly passing is therefore focussed on. What is interesting is the way that this "mouse called time" is constantly "nibbling," but we are unable to detect the passing of time. The third stanza says that only wise individuals such as "seers" and "sibyls" are able to actually be aware of the passing of time, and the final stanza explores how the origins of time are lost in obscurity:
And whence or how he comes
And how or where he goes
Nobody now remembers,
Nobody living knows.
Time is therefore placed outside of the sphere and understanding of humanity. This extended metaphor is effective because so often rodents like mice live their lives completely separately from us, and only every now and again are we aware of them and how they impact our lives. Thus this is a suitable comparison because the major aspect of time that Robert Francis is focusing on is its imperceptibility.