@yusha To be more precise (if you care haha), ferritin is a protein as well as a storage molecule, as is another protein called transferrin (the body developed this mechanism not only to move iron in and out of cells, but also to sequester it from bacteria that also utilize iron). Ferritin's production is increased when you're taking in a lot of iron (so it can compensate and store it).
High ferritin levels: Storing too much iron (as in hemochromatosis), alcohol abuse, rheumatoid arthritis, liver disease, hyperthyroidism, and (sometimes) cancer.
Low ferritin levels: Iron deficiency anemia (there are many types of anemia!) It could be caused by low dietary iron intake, poor iron absorption in the intestine (anything that destroys the surface cells of the intestine messes with this, eg. celiac disease), blood loss (eg. heavy periods), or pregnancy.
Iron-rich foods for us vegs: Beans, dark leafy vegetables (spinach), dried fruit (raisins, apricots), iron-fortified foods (bread, pasta, cereal), and peas. A multivitamin can also help, if your diet is lacking.