"Data values" refer to the type identification numbers used to internally distinguish between one block type and another.
Recall that in any real computer system, everything is internally represented as numbers; Starmade is not an exception. When a computer is running Starmade and needs to record what type of block is in a particular location, the computer records the data value that represents that block type. If the block is a Ship Core, then the computer records "1" as the data value. If the block is a SD HCT xm3.4 Power block, then it records "2" as the data value. A complete list of the data values may be found at the Blocks article.
Data values are seen in many games as a quick and easy way to refer to every item in the game by assigning a number to each block or item. It is essential to spawning in items when you are an administrator of a server.