You can select files in a view window by hovering over the view with your mouse. The file under the mouse cursor is selected and marked by a highlighted rectangle; the folders in which the file resides are similarly marked. The name and size of the selected file are shown underneath the view.
You can change the selection from the file to one of the folders in which it resides (this way, you can for example quickly find out the size of specific folders). You can do so using either the scrollwheel of your mouse, or using the "[" and "]" keys on your keyboard. After you have moved the selection to a level in the file hierarchy, it will remain at that level until you explicitly move it up or down. If you move the selection back down again to a file and go a little beyond it, then the selection will stick to files, as it did originally.
Clicking inside the view locks the selection so that it does not change when you move the mouse. Click once more, anywhere in the view, to unlock the selection.
When the selection is locked, you can change the folder that is shown in the view. By clicking the Down button, you descend into the first subfolder on the path to the selected file. You can keep going down, until you've reached the file itself. Clicking Up will take you back up, one level at the time.
When the selection is locked, you can also show the selected file or folder in Finder using the Reveal button. There you can examine it, rename it, delete it, etc.