Every time you launch lepton-schematic without arguments in a terminal, or
from your window manager menu, a new untitled schematic page is
created. Use File → New within lepton-schematic if
you want to create another page.
The default page name for new pages is formed as a filename prefix defined in the configuration key default-filename and a serial number which is incremented every time a new page is created, e.g. untitled_3.sch. Usually, a title block symbol is inserted, see Title block symbol.
Within the program, you can open an already existing schematic using the menu action File → Open... In the opened dialog box select the file you need and click the button ‘Open’ to actually open it, or ‘Cancel’ to cancel with it. At the bottom-right of the dialog there is a filter button. With it, you can change view to see schematics or symbols only, both schematics and symbols, or any files in case you want to look at some other stuff in the selected directory.
If you want to open a file you worked on recently, click the menu item File → Open Recent. The submenu contains several file names. You can configure how many files should be shown there using the configuration key max-recent-files.