When someone has died of natural causes it usually means that the person died due to heart failure, stroke, kidney failure, liver failure etc.
Basically the person could have died just because something went wrong in their body especially due to old age.
If a person is over 70 and dies at home or in a care home etc then it's usually listed as the person dying due to natural causes.
As our bodies get older our organs etc start to wear out and eventually they stop working such as our heart and when our hearts fail to pump blood then we die.
That heart failure would be a natural cause of death.
Murder would be a cause of death if someone killed the person but if they die naturally then it would be a death by natural causes.