UML (Unified Modeling Language) Tool is a software application used by software developers and system architects to create graphical representations of software systems, processes, and structures. It helps in designing, documenting, and visualizing software projects through various diagrams and notations used to represent different aspects of a system. UML tools are essential in software development processes as they help in improving communication and understanding between developers, project managers, and stakeholders involved in a project. They are used to create blueprints for software projects, which aid in ensuring their effectiveness and efficiency.