DigitBlock is an option for NumberForm and related functions which specifies the maximum length of blocks of digits between breaks.
The default setting is DigitBlock -> Infinity, which specifies that no breaks should be inserted.
DigitBlock -> n inserts a break every n digits.
DigitBlock -> nleft, nright inserts a break every nleft digits to the left of the decimal point, and every nright digits to the right of the decimal point.
The setting for NumberSeparator determines what string should be used at each break.
See The Mathematica Book: Section 2.8.7.