Dense trees are forested areas where the canopies of the trees are touching. Dense trees shelter the snow from the effects of wind and sunlight, they modify the snowpack by shedding snow (think tree bombs), and they inhibit the growth of surface hoar.
In some cases, dense trees also work to anchor the snowpack in place, limiting the potential for a slab to dislodge from the snowpack. Traveling in dense trees can be a good choice in periods of elevated avalanche danger.