They mean they're very scared of something — scared enough that they've turned to stone and cannot move! If you ever find yourself petrified, you may be standing as still as a tree in a petrified forest.
Petrified wood is what scientists call the fossilized remains of trees that have turned into stone through a process called "permineralization." This process occurred underground when, millions of years ago, wood became buried under sediment (like mud).
Sediment initially preserves the wood by insulating it from oxygen, which usually starts the decomposition process. Over time, all the organic (living) materials in the wood slowly decay and are replaced with minerals. These minerals are usually silicates (like quartz).
Petrified wood is different than other types of fossils. Most fossils are impressions or compressions. Petrified wood, on the other hand, is a three-dimensional fossil that retains the original structure of the wood.
Many examples of petrified wood contain a wide variety of bold, beautiful colors. Although pure quartz crystals are colorless, other elements — including manganese, iron, and copper — often find their way into sediment during the process.
These “contaminants" can give the resulting petrified wood many interesting colors, including black (carbon), blue (cobalt), green (oxidized copper), red (iron), yellow (manganese oxide), and pink/orange (manganese).
Petrified forests can be found all over the world. Some of the most famous sites in the United States include Petrified Forest National Park in Arizona, Petrified Wood Park in South Dakota, Mississippi Petrified Forest in Mississippi, and Escalante Petrified Forest State Park in Utah.