This is something you should experiment with if you are not already familiar with this kind of mouse behavior. In addition, rapid double and triple clicks with the left mouse button can be used to select the word the cursor is on or the line the cursor is on. A single click with the first button just moves the text cursor to the location of the mouse. Command and menu buttons always have a red background and white text. These always indicate things you can click the mouse on to have some action performed.
Command buttons carry out an action immediately, while menu buttons pop up a menu of options.
There are a number of occasions when Echo will need to get a file name from the user. To do this, it will display a file selector window. This window has various components. Often simpler, is to use the mouse to select the file you want. You can select a file in the scrollable list by simply clicking on it with the mouse.