Question

what does it mean by a blood moon

Answer 1

The full moon nearly always appears coppery red during a total lunar eclipse. That's because the dispersed light from all the Earth's sunrises and sunsets falls on the face of the moon at mid-eclipse. Thus the term blood moon can be and probably is applied to any and all total lunar eclipses.

Answer 2

The moon appears blood red during a total lunar eclipse.

A "blood moon" happens when Earth's moon is in full eclipse. While it has no special astronomical significance, the view in the sky is striking as the usually whiteish moon becomes red or ruddy-brown