Data pads
From Halopedia, the Halo wiki
The Data pads are hidden computer pads encountered throughout the Halo: Reach campaign. Similar to the Terminals found in Halo 3, the pads contain their own, self-contained story, presented as exchanges between members of a mysterious "Committee of Minds for Security" consisting of several Artificial Intelligence constructs.
A total of 19 data pads can be found at various locations in the campaign. Some campaign levels could have more than one data pad. Each data pad is numbered; letters or words are replaced with numbers in each data pad message representing the number of the data pad. Data pads numbered 1-9 can be found on any difficulty settings whereas data pads 10-18 can only be found on the Legendary difficulty settings.
For a walkthrough of how to obtain the data pads, see Data pads/Walkthrough.