In the main window's menu choose File | Update or the corresponding button on the toolbar.

PopMan then connects to the POP3 server that you specified during account setup and starts loading emails (small emails are loaded completely, big emails are loaded partially).

In the left pane of the statusbar the current operation is displayed (Connecting, Login, Loading, ...). If something goes wrong (password denied, connection failure, ...), an appropriate error message is displayed in the rigth pane of the status bar:

If there is not enough space to display the entire error message, you can easily make it visible: Move the mouse cursor into the area and wait a short moment:

The emails are loaded one by one and are displayed in the main window's listview.