Xie's Mod can make use of Minecraft Forge spritesheets to specify graphics for textures and items in the game.
What is Forge Edit
Minecraft Forge is an "API" — Application Programming Interface. It is essentially a Minecraft modification that "exposes" a large number of compatibility features within the Minecraft source code, allowing modders like Xie the ability to write code that is compatible with other mods. It also has lots of other neat features, too.
What is a spritesheet (or: why use Forge) Edit
Normally, Minecraft provides just two sprite sheets: one for terrain, and one for items. Modloader (one of Xie's Mod's dependencies) revolutionised Minecraft modding by allowing programmers to add icons/textures to these sprite sheets, but unfortunately there is only a finite amount of room left over from the amount of sprites already included in standard Minecraft. Once you use up the last sprite, that's it -- expect it to fail, crash, or worse if you try to add another. Although terrain and items sprite sheets are separate so you have room for new blocks and new items, it is all too easy to use up all the room sooner than you might expect, especially when using mods other than Xie's Mod.
Forge fixes that problem. One of Forge's very first features when it was introduced was the ability to have a theoretically unlimited number of sprite sheets. As long as your computer has enough video memory, you can load a nearly infinite number of sheets containing 16 rows of 16 icons at your desired Minecraft resolution. This revolutionised a lot of Minecraft modding and allowed such giants as BuildCraft, IndustrialCraft, and RedPower not only to work, but also to work together. Instead of being limited by the amount of textures available, a small number in the dozens, suddenly the only limit on Minecraft modding was on the number of block and item IDs available. For blocks this is still a small number in the range of a hundred; for items, however, this is a number in the thousands. It didn't take long for Minecraft modders to capitalise on this freedom!
If Forge is not installed Edit
Because Xie's Mod does not explicitly require Forge to be installed for the single-player version, it is designed to "fail gracefully" if it is not included. Instead of crashing by trying to reference sprites off of a non-existent spritesheet, it will simply default to referencing sprites off of the vanilla Minecraft terrain or item sprite sheets. This does, however, produce an obvious visual bug: although the mod will still work, the objects will take on the appearance of whatever icon is on the vanilla spritesheet instead!
For instance, if the Forge sprite sheet that a content pack uses has a smoking pipe as object number 25 on the grid, if Forge is not installed it will look like a bushel of wheat. It still works, but it looks silly to be smoking wheat. Perhaps it would be misinterpreted by law enforcement officials, also.
How to tell if a content mod requires Forge Edit
If you are unsure whether a content pack requires Forge, do a search within all of the .xie files for the keyword "texFile". If anything comes up, the content pack requires Forge!