In Java, a frame is a top-level window with a title and a border. It is a container object that can hold other GUI components like labels, buttons, and text fields. Frames are used to create window-based applications and are an essential part of building graphical user interfaces (GUIs) in Java.

Frames can be created in several ways, including by creating an object of the Frame class, by extending the Frame class, or by using Swing components inside the main method. The JFrame class is a type of container that inherits the java.awt.Frame class and is commonly used to create frames in Swing.

Frames have various features, including a title, a border, and buttons along the top border. They can be customized using various methods, such as setBackground() to set the background color, setIconImage() to set the image to be displayed as the icon for the window, and setExtendedState() to set the state of the frame.

In summary, a frame in Java is a top-level window that can hold other GUI components and is used to create window-based applications. It has a title, a border, and various customizable features.