Things leftover from doing something else.
Driftwood is wood, usually branches from trees, that has fallen into the water and then drifted around for a while before washing up on a beach that may be a long way from where it fell into the water. Driftwood is often polished smooth by the waves and water, and bleached to a dull grey.
Dust is very fine dirt and lint that drifts through the air and settles on surfaces in your house, making them dull and grey. Dust needs to be cleaned up regularly with a soft cloth. The picture is of some cables from the inside of a computer. They are very dusty.
When there is a storm that picks up a lot of dirt in the air, it is called a dust storm.