Redis is a very sophisticated data structure server, that can be used as a powerful in-memory database. However, if you look at redis as database per-se, it only has primary keys. There is no native way to ask redis for something like "what are the names of users over 18 who have visited my website yesterday?".
If you're familiar with traditional relational databases, this is usually done by creating an index on the relevant columns in your table, allowing to efficiently add complex WHERE clauses to your query. Such an index is called a Secondary Index.
While it is possible (and done by many many people) to implement these indexes on top of redis manually, doing them right and in a performant way is hard.
