Infusions are water soluble particles with 2 components: a fine permanent dye colour pigment mixed with a walnut crystal particle. The 2 react with water at different rates - you will see instant colour, and then a brown base will slowly develop as the walnut crystal dissolves more gradually.
This allows you complete control over the colour-to-brown ratio depending on how much water you apply, and how fast you blot or dry the surface with a heat tool. Suitable for use on paper/ fabric, wood, through stencils, into gel mediums ie most porous surfaces, but also nice when used with both wet and dry paint.
How to set up your infusions bottles for use is explained clearly in this video by Scrapcosy on YouTube.