For a fossil to form it requires special conditions. The original organic matter (e.g. animal or plant) must be in conditions where it doesn’t completely rot down, which often means an environment with no oxygen such as in a bog. Once there, the mud around it will turn to rock over time, and the minerals from the rock will seep into the organic matter to turn that into rock as well. This is how we get fossils.
In the case of this fossil fern, it formed in the Carboniferous period, and the dark shape of the fern leaves is basically coal. Coal, in fact, comes from plants like these, which is why coal is known as a fossil fuel.
Can you find a fossil you like in Manchester Museum?