Scripts, textures, and models are some examples of source data files.
Source data are the raw assets which have not yet been converted into tags useable by the engine. They live under the data directory of your mod tools and include uncompressed TIFF textures exported from your 2D software, models and animations you have exported from your 3D software, and level scripts. In most cases, assets within data will have a corresponding tag under the same folder layout in tags.
Converting these assets into tags is called importing or compiling and is usually done by Tool. The types of source data formats used depends on the game; some are common but others are generational. H1-H3/ODST differ between each other, but differ even more compared to later games like Reach, H2A, and H4.