To be included in Wikidata, items need to meet the project's criteria for inclusion. The term "notability" is inherited from Wikipedia, where it has a more restrictive meaning. If the item you're trying to describe actually exists — either materially or conceptually — and it can be clearly distinguished from similar items, and you can have reliable sources that you can use to describe it, then it probably meets the criteria for inclusion. Recall that you need to be able to describe the item using the item-property-value triples, and you need sources to support those statements. If you're having a problem finding sources for something, it may not be a viable item. Items that fill a structural role on Wikidata are also allowed, even if you don't have sources that describe them in any depth. For example, you may not be able to find any information about a person beyond the fact that they're listed as the author of three books which have items. Creating an item for that author fills a structural need, because it makes it possible for queries to connect those three books together.