Subitizing is the ability to quickly recognize the number of items in a group, without counting each time. While this is typically done with smaller numbers (between 1 and 4), it can be done for larger numbers with practice. This is especially true if using common patterns, as in the number 6 on a die.
There are two different types of subitizing. Perceptual subitizing is used with smaller numbers, especially those in patterns. This is typical when reading a roll of a single die, since the numbers are indicated with commonly used patterns of dots. Conceptual subitizing is used for larger numbers. This involves the brain breaking the group into smaller, recognizable patterns. Addition is then used to find the total without counting.