Introspection

AnyDice provides two operations that allow you to query a value about its contents.

Length

You can perform the unary # operation to get the length of a value. What that means depends on the type of value involved.

Sequences: The length of a sequence is equal to how many numbers is contains. So #{2,4,6} yields 3.
Collections of dice: The length of a collection of dice is equal to the number of dice it contains. So #(3d6) yields 3. Note the parentheses, without them it would be intepreted as (#3)d6, which would yield 1d6.
A single die: A single die is considered to be a collection containing one die, so its length will always be 1.
Numbers: The length of a number is equal to how many digits it has.

Accessing

Using the @ operation, you can retrieve individual numbers stored inside a value. You can either provide the position of a single value to retrieve, or a sequence of positions, in which case the retrieved values are summed. If you ask for a position that does not exist, the result for that position will be 0.

Sequences: For sequences, the number at the indicated position will be retrieved. So 1@{2,4,6} yields 2, 3@{2, 4, 6} yields 6, {1,3}@{2,4,6} yields 8, and {1,2,2}@{2,4,6} yields 10.
Collections of dice: When accessing collection of dice, the dice will be order from highest value to lowest value. So 1@3d6 will select the highest rolled value, while 3@3d6 will select the lowest rolled value. {1,2}@3d6 will sum the highest two of the rolled values, discarding the lowest rolled value. The result of the operation is a new die containing the selected values of all possible rolls and their appropriate odds.
A single die: A single die is considered to be a collection containing one die, so only the first positions will contain a useful value.
Numbers: For numbers, the digit at the indicated position will be retrieved, starting at the most significant – the leftmost – digit. So 1@246 yields 2, while {1..3}@246 yields 12.

If you want to, you can configure AnyDice to reverse the positions of dice in collections and of digits in numbers.

set "position order" to "lowest first"

output 1@3d6 named "lowest die"
output 1@246 named "least significant digit"

set "position order" to "highest first" \ the default behavior \

output 1@3d6 named "highest die"
output 1@246 named "most significant digit"