Queries are essentially powerful Filters. Queries allow you to decide what fields or expressions are to be shown and what information to be sought. Queries are usually based on Tables but can also be based on an existing Query. Queries allow you seek from very basic information through to much more complicated specifications. They also allow you to list information in a particular order, such as listing all the resulting records in Surname order for example.
Queries can select records that fit certain criteria. If you had a list of people and had a gender field you could use a query to select just the males or females in the database. The gender field would have a criteria set as "male" which means that when the query is run only records with "male" in the Gender field would be listed. For each record that meets the criteria the you could choose to list other fields that may be in the table like first name, surname, phone number, date of birth or whatever you may have in the database.
Queries can do much more than just listing out records. It is also possible to list out totals, averages etc. from the data and do various other calculations. Queries can also be used to do other tasks, such as deleting records, updating records, adding new records, creating new tables and creating tabulated reports.