I believe that would indicate you are below the minimum safe altitude - for VFR flying this isn't necessarily an issue, but worth understanding why it's warning you, as while e.g. if it's due to a mast near your track which you will be able to see and avoid that's probably not an issue, if it's because the ground rises up you might either end up violating rule 5 (by e.g. flying too low over a built up area), or worse fly into the ground if it rises significantly...