In Houdini, we have three states that revolves around the viewport.
Whenever we invoke a state, on the top toolbar, we are going to see information associated with invoked state. For example, if we would invoke selection state or selection tool, on top of the viewport, we would see the toolbar with different icons, which we can use to enable or disable certain features of selection tool or any other tool that is invoked.
This approach is similar to one in Photoshop where on the left, we have tools to pick, and on the top we have settings which are associated with those tools.
Understanding states is most important thing in Houdini as ultimately this will allow you to work fast and avoid all potential confusion which people encounter when there is a lack of basic understanding of these basics.
To escape these states, we can press escape on our keyboard.
Once you understand the concept of the states you can proceed to modify shortcuts and radial menus which will introduce another layer of complexity, but ultimately make things easier and faster.