A mash bill or grain bill is the whole of the different malts, grains, and adjuncts that make up a beer. The mash bill is very important when designing a beer recipe, as it is primarily responsible for the potential original gravity, color, and diastatic power of the beer.
Diastatic power or enzymatic power is the measurement of how much starch-converting enzyme a malted grain contains. It is shown in degrees Lintner. If your mash does not contain an adequate amount of diastatic power, you will not convert a high enough portion of the starch in your grain bill to sugar, which will translate to a low brew house efficiency and lower than expected starting gravity.
To be safe, you should aim for an average of at least 75 degrees Lintner for your total grain bill. For the most part, light base malts are used to make up the real diastatic power of a mash. Caramelized, toasted, and roasted malts have diminished diastatic power due to excessive heating. Generally, the darker the roast of the malted grain, the less diastatic power it will have.