It is a trace fossil - a sign left by an animal rather than the animal itself.
In this case, it is a burrow, filled with pretty much the same sediment as that surrounding it. The slight difference in colour is probably due to subtle differences in mineralogy/chemistry. That's due to the animal ingesting the sediment, digesting the food it contained (minute organic material), and excreting the remainder behind it.
Quite what the animal was, we can't say in this case.
In other, rarer, cases, the animal is found as a proper fossil within the burrow. That's obviously very interesting to palaeontologists. It helps with piecing together the whole gamut of traces and animals (and plants - they can leaves traces, too). But however much we learn, some ambiguity will remain. That is, some burrows look identical despite being made by different creatures.
With proper fossils (not so much trace fossils), in general, we can use the location to narrow the range of possible fossils. But that really needs the specimen to be in situ, not loose on a beach or in a stream bed. In northern England, loose material can have come from a long way away, having been transported by glaciers.