There are already environments on Earth that are similar to this, like the floors of rainforests.
I grew up close to some natural rainforest areas and 'shadow' is a reasonably good term for the light levels you get while walking through a dense rainforest. The canopy blocks out a great deal of the sun, which is why the trees in rainforests have already evolved to grow so tall; they need to compete with each other to get the most light out of the 'canopy' above the ground.
Some plants actually thrive in such an environment; you get a lot of ferns for instance that don't mind being in shaded environments and actually burn in full sun. Plants like 'elephant ears' evolve to have very large, broad leaves that take advantage of what sunlight they can get a hold of, and as such would burn in full sun because their leaves have such a large surface area.
And then you have mushrooms. These aren't animals, but they're not plants either and don't photosynthesise, so the presence of sunlight isn't a problem for them. What they do need though is plenty of organic material from which they can draw energy and material with which they grow.
Animal life is a bit more complicated because it generally relies on plants to survive. Most of the insects in rainforests for example thrive on the humus, or plant debris on the rainforest floor that starts to decompose and compost. Then, the larger animals live on them.
In your world, rainforests may be an option in some places where the shadow isn't particularly high, and trees can grow above it and do. In most places however, your plant biome will likely be fern prairies that contain additional broad leaved plants designed to maximise energy absorption. In such cases, there may well still be a humus layer and hence insects, but what you're unlikely to see would be fruits and flowers.
Why? Well, fruits represent a massive store of chemical energy (sugars and the like) that the plant invests in it's propagation. The trouble is, if energy is scarce, you simply don't reproduce like that because the energy required to produce fruit is a much larger percentage of the available energy, and hence the investment represents an impediment to survival.
So, any animals in such a biome are likely to be predators, starting with the consumption of the insects that work the humus layers in your fern prairie.