There are five ways for a fossil to be form:
-Preserved remains, where the original material is preserved, such as insects preserved in amber.
-Permineralizarion, where bone, wood, or shells are buried in sediment and are replaced by minerals.
-Molds and cast, where hard structures such as bones or shells leave an impression in the soil with their shape, which is filled by other sediments when the original material dissolves.
-Replacement, where the original shell or bone dissolves and is replaced by a different mineral, like shells originally made of calcite replaced by pyrite.
-Compression, when the remains are subject to high pressure and they leave a dark mark or imprint. This occurs very commonly for fossils of leaves and ferns on the rocks.
In the question, the image shows a dark fossil of a fern, most likely a compression fossil.
If we look at the dinosaur from the previous question, we can see that is a fossil formed by permineralization.
Compression differs from permineralization in several ways, including that permineralization occurs to hard structures, such as bones or wood, while compressions fossils are usually leaves. Another difference is that in permineralization the remains are buried in sediment and later replaced by other minerals, while in compression fossils are imprints caused by high pressure over rocks.