Symbolism

In the Dutch golden ages (1575 - 1675) a lot of painters were painting Vanitas paintings. These are still life artworks which show various symbolic objects designed to remind the viewer of their mortality (the fact that humans don't live forever) and of the worthlessness of worldly goods and pleasures.  

Skull: Death, this is a clear memento mori message or the transience of life, a universally recognized symbol of death.

Watch or hourglass: time is limited and is passing, therefore, use it wisely.  See a typical one-handle XVII century watch at the lower right of the photo.

Books: Human knowledge and its temporary nature.

Shell: they were normally exotic ones not commonly available in the Netherlands. They were a symbol of the vanity that comes with wealth, as these were exotic items at the time, only a very wealthy person would have one of those.

Insects, decaying flowers: transience of life. They were inserted in paintings depicting

expensive objects as a reminder that life is temporary and moral considerations deserved

more attention than material things. 

Broken or tipped over glassware: transience of life or life is fleeting.

Musical instruments: indulgence of the senses as a luxury. Sometimes they are present as artistic inspiration, as music would inspire artists.

Silk or velvet tablecloths: vanity, as these were expensive things. Silk being the ultimate fabric material and purple the most expensive dye, hence the Roman emperors wore purple tunics. 

Jewelry, clothes or mirrors: remember the temporary nature of beauty, wealth and wisdom. Earthly riches are temporary and therefore life should be carried out according to the modestytraditions that were in place at the time.