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import mods.ic2.AdvRecipes;

There are several types of recipes:

Shaped Recipes are recipes, where it matters, which item goes into which slot. For example, you can't just arrange 7 different sized stacks of iron ingots in any order to create iron leggings. The shape matters, thus it is a shaped recipe.

Shapeless Recipes are recipes, where only the items you put in the crafting grid matter, whereas the shape is of no importance. For example, blue and yellow dye create green dye. This recipe doesn't care about where you put which item.

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mods.ic2.AdvRecipes.addShaped(output,inputs);

This creates a shaped recipe for output using inputs as Ingredients.

output is an IItemStack
inputs is an IIngredient[][] (see below)

inputs is a 2 Dimensional IIngredient Array.
So the recipe for Iron Leggings would be written as [[iron,iron,iron],[iron,null,iron],[iron,null,iron]]
If that looks to confusing, try splitting the arrays up into one array per line

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val iron = <minecraft:iron_ingot>;
val leggings = <minecraft:iron_leggings>;

AdvRecipes.addShaped(leggings,
 [[iron * 5,iron * 7,iron * 5],
  [iron * 3,null,iron * 3],
  [iron,null,iron]]);

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mods.ic2.AdvRecipes.addShapeless(output,inputs)

This creates a shapeless stacked recipe for output using inputs as Ingredients.

output is an IItemStack
inputs is an IIngredient[] (e.g. [minecraft:dye:1,minecraft:dye:2])

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mods.ic2.AdvRecipes.addHiddenShapeless(IItemStack output, IIngredient[] ingredients);
mods.ic2.AdvRecipes.addHiddenShaped(IItemStack output, IIngredient[][] ingredients);

This creates a shaped or shapeless stacked recipe for output using inputs as Ingredients that is hidden.