Cellulose is a compound that gives plant cells their rigidity. We humans cannot digest it, so it provides no calories and is a source of fiber (we get it in fruit, vegetables, and nuts). It is also called wood pulp, the main ingredient for paper.
Cellulose gets a lot of hate from drama blogs freaking out and telling you that there is sawdust in your cheese. While technically true, this is like saying there’s over a gallon of thick red liquid inside of you, carrying poisonous CO2 and possibly viruses around. Yeah there is, we call it blood, and we need it.