A cruet is a small container for condiments, oil, vinegar or sometimes soy sauce. They are usually glass or ceramic and have a stopper or pourer to control the flow of the liquid inside. You see them on dining tables in restaurants, cafes and even at home. They let you add flavour to your meal to your taste.
Cruets are big in hospitality. They give guests control over their meal's seasoning, for different tastes and dietary needs. For restaurant staff, it simplifies service as you don't need to ask for individual condiments. It also adds to the table presentation, a touch of class to the setup.
Let's say you're a server at an Italian restaurant. It's dinner time and you're setting up tables for incoming guests. You put cruets filled with olive oil and balsamic vinegar at each table. As you serve a table of 4 their appetizers, one of the guests asks about the cruets. You tell them it's high quality extra virgin olive oil and aged balsamic vinegar for dipping bread or drizzling over salads. They appreciate the info and the ability to customise their meal, and you get better tips.