Your graphical desktop environment (most likely KDE or GNOME), in an effort
to behave like Windows, probably has its focus policy set to “click to focus.”
This means for a window to get focus (become active) you need to click on it.
This is contrary to the traditional X behavior of “focus follows mouse” which
means that a window gets focus by just passing the mouse over it. The window
will not come to the foreground until you click on it but it will be able to receive
input. Setting the focus policy to “focus follows mouse” will make the copy and
paste technique even more useful. Give it a try. I think if you give it a chance
you will prefer it. You will find this setting in the configuration program for your
window manager.
But still I am not able to know the difference, setting to 'focus follows mouse', still I have to click the folder to open it, and then what does it mean (above)?