What does Keep your head down and your powder dry mean?

The phrase comes from a saying attributed to Oliver Cromwell (1599-1658), and the powder is, of course, gunpowder. During his cold-blooded and savage Irish campaign of 1649 he is said to have concluded a speech to his troops, who were about to cross the River Slaney before attacking Wexford, with the rousing Words, 'Put your trust in God, my boys, and keep your powder dry.'

However, it seems likely that the phrase was coined later by the Anglo-Indian soldier Valentine Blacker (1738-1823) and gained the Cromwell attribution through being quoted in 'Oliver's Advice' in E Hayes's Ballads of Ireland (1856).