Imagine there is only the bass clef there. Middle C is, as usual, shown on its own ledger line. But now, the notes need to go higher than that, still be played by the left hand, so still be written using bass clef.
More ledger lines are used above the one for middle C. They are always a third above the last one. So here, the dot on the second ledger line is E. The note in the space above, which would just rest above the E line, is F, and if a G was needed it would have its own third ledger line.
That line for the E is actually the same as the bottom line in treble clef, but putting that E dot in the treble clef would mean it gets played with th eright hand - not what the writer wanted.
The same phenomenon works the opposite way too. Low notes on the treble clef can have ledger lines of their own. Imagine an A under middle C. That could be written on the second ledger line down - corresponding to the top line of the bass clef.
EDIT: When introducing students to the grand clef, I use the idea that once there were eleven lines, but it was too unwieldy, so it was split into two, with middle C given its own floating ledger line. So if one of the staves needed more lines or spaces, it continued with ledger lines in the appropriate direction. Which includes going above the treble and below the bass clef. Makes it easy to understand.