The dry matter level of a nutrient is the percentage there would be in the food if all of the water (also called moisture) was removed.
The nutrient levels given on the packaging, on the other hand, are 'as fed' meaning they represent the percentages of the nutrients in the finished product without any water being removed.
While this may seem like a trivial distinction, it is important to note that in order to compare the nutrient levels of two foods with different moisture levels, you really need to look at the dry matter nutrient levels since 'as fed' values don't tell the whole story.
Let's look at a quick example:
Food 1: Dry food. 8% moisture, 25% protein.
Food 2: Wet food. 70% moisture, 15% protein.
To the untrained eye, the wet food looks like it's quite a lot lower in protein than the dry but, in reality, it contains almost twice as much! Here's why:
The dry food is only 8% moisture. That means, in 100g of food, 8g is water. This leaves 92g of 'dry matter', 25g of which is protein. As a percentage that is 25 / 92 x 100 which comes to 27%. This is the 'dry matter protein' value.
The wet food, on the other hand, is 70% moisture meaning that 100g of food only contains 30g of dry matter. Of that 30g, 15g is protein. 15 / 30 x 100 = a whopping 50% dry matter protein.
Doing all that maths for every ingredient in every food your interested in can be very time consuming so to make things easier we have added dry matter nutrient dials to every food we feature and when you search for foods with certain nutrient profiles, it is always done according to the dry matter values.