The word grave has two completely unrelated meanings:

1. Serious and solemn, requiring careful thought. When a doctor delivers grave news about someone's health, her expression shows the importance of what she's saying. A principal might speak in a grave tone when addressing a serious problem at school. The word suggests weight and significance: a grave mistake is one with serious consequences, and a grave decision is one that shouldn't be made lightly. You can hear the seriousness in phrases like “a matter of grave concern” or “a grave threat.” When something is described as grave, it demands respect and careful attention.
2. A hole in the ground where a dead person is buried. Graves are usually marked with headstones that show the person's name and the years they lived. People visit graves to remember and honor those who have died. You might see graves in a cemetery, sometimes decorated with flowers. The word can also mean death itself, as in the phrase “from the cradle to the grave,” meaning from birth to death.