Okay.
Firstable, there are 3 kinds of symbiosis : parasitism, mutualism and commensalism.
If u want to understand it ezly.
Paratism=one of them good others are super-bad. e.g.)tapeworms, lampreys...
Lichens are made up of algae and fungus. The algae makes food through photosynthesis for the itself and the fungus whereas the fungus anchors plant and provides itself and the algae with moisture.