Sentience being inherently subjective means we can't really detect it directly, we can only infer it, so this can be a tricky subject. That said, it is basically just any sort of awareness of one's environment, and in that regard it definitely comes in "tiers" or more like along a spectrum. We observe evidence of sentience in all sorts of things, pretty much anything with a central nervous system has some level of sentience, as that brain is there to process the information being gathered from the sense organs.

A blind, deaf worm and a tiny simple brain has a relatively low level of sentience, but even they can respond to feeling pressure, temperature, exhibits signs of feeling pain when injured, etc. That may be on the low end of the sentience spectrum compared to us, who can also see, hear, smell, and can even experience abstract or non-external, stimuli through things like our imaginations and self-awareness.

Basically the more sense organs an organism has, and the more sophisticated the brain is to process it, the more sentience the organism appears to have. There are exceptions, lots of animals exist that also exhibit signs of some form of awareness that don't have any recognizable central nervous system, but it's hard enough to measure in organisms with brains and recognizable sensory apparatus, so we'll leave those weirdos out of it for now for the sake of simplicity.

Even within a singular organism, their sentience can come along a spectrum, depending on how well their brain or sense organs are working at any given time. If you're really alert, like wired on caffeine, then your awareness is operating at a high level, whereas if you're very sleepy, or impaired, or your senses are numbed by an anesthetic, or if you're outright knocked unconscious, your awareness is obviously impaired and reduced. We don't typically think of sentience in this manner, but I think it's a fair point to raise to help illustrate the point.