modal means there is no move to next step in vb.(e.g)msgbox

modaless means we can easily move to other windows (eg)menus

Modal - Is a type of message box where you need to respond to it by clicking any of the buttons displayed before responding to any other windows in the background.

Modeless - Is just the opposite i.e. you are allowed to respond to other windows first and need not attend the current displayed message box.

A modal form or dialog box must be closed or hidden before you can continue working with the rest of the application.

On the other hand Modeless form or dialog box not necessarily needed to close for continue working with another application.

Simply
If you write down this code to show(Activate) Form1 Form1.Show vbModal

You can not access any other form untill you close(Unload or Terminate) the Form1.