The wood fibres have been crushed rather than being broken by a scratch. The technique to take out the dent is to steam it by pressing an iron on to a damp cloth against the dent. This can spring the fibres back into their original shape. As mentioned above though the results are unpredictable, this can ruin the varnish. I did this to a solid Alder table and it needed sanding and refinishing once completed but it took out the dent really well. It must be solid wood for this to work.
Dave of Bexleyheath, October 2008