A dredger is a large shaker. It looks like an aluminum cheese shaker in a pizza parlor. It has large holes in the top, and may have a domed or flat top. It may have a handle or not. This device allows a cook to sprinkle flour on to a piece of meat, on top of dough, or on anything that needs a coating of flour.
Dredging meat helps keep it moist while frying. Seasoning can also be added to the dredging flour, which then transfers to the meat, as well. Using a dredger means the cook does not have to handle the meat as much, which translates to a lower risk of contracting food-borne diseases and results in a higher level of food safety.
This increase in food safety is seen especially when the cook is doing something like frying chicken. Many cooks dip the chicken pieces in egg and or milk, and there is a good opportunity for cross-contamination with the flour mixture. A dredger takes care of that possibility by never having the whole batch of flour contact the chicken. If a heavier flour coating is desired, the cook just shakes more flour out. Using one is a much safer way of preparing raw meat for cooking.
Any kind of shaker with large holes can be used as a dredger. The larger the holes, obviously, the more flour that will be shaken on to the food. Cheese shakers are available everywhere, for under $10 U.S. Dollars. A utensil actually called a dredger is usually made of aluminum and will retail for $15 to $30 USD.