Data is a word used in everyday life as meaning pieces of information, usually gathered by scientific methods, or at least pseudo-scientific ones, and meant for analysis and processing. This loose definition is applicable to data in computer terms, but it has a deeper significance in this field as well.
All a computer truly is is a machine that performs operations on information. That information is data, be it quantities, characters, symbols or any aggregation of those types. There would be no computing without it! Data is constantly being stored, transmitted and transformed by operation for more uses than you could probably imagine. On a fundamental level, it takes the form of electric signals and magnetic, optical or mechanical records that the computer manipulates which can represent information humans are able to understand clearly.
Coded instructions that make up computer programs are also just essentially a sequence of data, so all programs are data. However, program file types are typically different to that used for ordinary data so we tend not to refer to programs as data, in order to avoid confusion and preserve the distinction between the written instruction code which is technically a form of data, and the actual carrying out of that code by the CPU, which is not. Therefore, all files that are not executables (programs) are the ones we freely call data files. Information stored in memory is clearly also data, only this data is cataloged via memory references, which are another kind of data. Here, we see digital data reduced to key/value pairs to allow programs easy access to the physical computer memory storage available. As you can see, data both instructs the operation of a computer as well as provides the memory information to be operated on.
An alternative definition of 'data' in computer science is also 'binary files', called that to distinguish them from alphanumeric text that can be read easily by humans.