Protects your world seed by altering feature and structure placement, preventing seed cracking.
When generating worlds Minecraft builds the basic surface shape, then applies biomes and finally adds surface decorations, which consist of features (trees, sugar cane, grass, ores...) and structures (villages, pillager outposts...).
Each feature / structure has its own seed, which is hardcoded in vanilla (to ensure a world with the same seed always looks the same). SeedGuard replaces these seeds with randomly generated seeds, making it impossible to derive the world seed from features, structures or surface rules.
SeedGuard changes the placement of features / structure in Minecraft, below is a simple demo of a world with and without the mod installed.