What is quick action in Salesforce?

Quick actions enable users to do more in Salesforce and in the Salesforce mobile app. With custom quick actions, you can make your users’ navigation and workflow as smooth as possible by giving them convenient access to information that’s most important. For example, you can let users create or update records and log calls directly in their Chatter feed or from their mobile device.
Quick actions can also invoke Lightning components, flows, Visualforce pages, or canvas apps with functionality that you define. For example, you can create a custom action so that users can write comments that are longer than 5,000 characters. Or create one that integrates a video-conferencing application so that support agents can communicate visually with customers.