HTML tag is used to embed another HTML document into the current one. It is a way to include external content directly on a webpage. It allows developers to show documents, videos, images, or any other type of content from one website to be displayed on another website.The syntax for using the tag in HTML is as follows:```html```Here, the \"src\" attribute specifies the URL of the content that will be displayed in the iframe. The \"width\" and \"height\" attributes define the size of the iframe.Let's say you want to embed a Youtube video on your webpage. Here's an example of how you can use the tag to do that:```html```In this example, we set the width and height of the iframe to 560 and 315 pixels respectively. The \"src\" attribute contains the URL of the video we want to embed, in this case, the Youtube video with the ID \"dQw4w9WgXcQ\". We also added some extra attributes, such as \"frameborder\", \"allow\", and \"allowfullscreen\", to improve the user experience.Overall, the tag can be very useful for displaying external content on your website. It enables you to present different types of content in a single page while also increasing the quality of the user's experience.