As we all know, tiktaalik is a popular transitional fossil. Its traits are significant as they're some of the earliest of their kind. It has a flat head with eyes on top, much like amphibians of the late devonian. It has wrist bones. It has spiracles for breathing air. It has robust pectoral girdles and a robust rib cage for lifting itself against the forces of gravity above water, much like amphibians of the late devonian. It also has an unfused skull and a neck for turning it's head while it's body remains stationary, which is something found in amphibians but not fish.
It is very much a tetrapodomorph with many traits of amphibians.
But it also has fins, gills and scales like a fish.
Which means that it was basically a hybrid between fish and tetrapods.
All the above aside, what makes tiktaalik more significant isn't simply its traits, but how, where and even "when" it was found.
In the fossil record, no land animals are found anywhere in precambrian, Cambrian, ordovician or silurian rock, nor anywhere in between. By the mid to late devonian, we find tetrapods/amphibians like salamander like species that walked on land.
So if evolution were true, of tetrapods evolved from fish, a species like tiktaalik ought to exist between the earliest formations of the devonian or by the end of the silurian at the latest, and the late devonian.
Before tiktaalik was found, Neil Shubin and his team knew this. So the scoured geologic maps for rocks of roughly the mid devonian to find rocks between fish and tetrapods where tiktaalik might hypothetically be found.
So they rented a helicopter trip to the Canadian Arctic where these middle aged rocks could be examined.
They originally started out searching marine devonian strata and realized that they needed to move inland (prehistoric inland) to the west, and they had to make their way to geology of a river bed/lacustrine origin. And it was there that some 10-15 tiktaalik specimen were found.
The reason that this serves as evidence for evolution is that it confirms the succession of fossils in accordance with genetic analyses of modern day life. Fish are genetically more similar to tetrapods than to any other animal of higher derivation, which means that it ought to follow, based on genetics, that tiktaalik ought to be present in the location in which it was later found. This is a prediction made with the understanding of descent with modification and common descent, and tiktaalik holds the feature that we might expect to be found in a particular place at a given time.
On a side note regarding the counter argument related to zachelmie polish trackways:
It should also be noted that polish zachelmie trackways have been reinterpreted as fish feeding traces. For those who like playing that card. Regardless, even if tetrapod trackways did pre exist tiktaalik, they'd still post date fish and pre exist all known tetrapod fossils, which would still fit in with the theory of evolution anyway. It has also been suggested that tiktaalik itself might even be responsible for such trackways even if they hypothetically were actually trackways and not feeding traces. And ultimately there are no zachelmie fossils discovered to date of any tetrapods, and so pointing them out is a weak counter argument to begin with as literally no bones even exist to make the counter arguments case.
https://www.tandfonline.com/doi/abs/10.1080/10420940.2015.1063491