The flame particle texture, is a texture for flames.
If you have a particle flame, then you can for example put a skull into the Particle texture, and all the particles coming from the flame will look like skulls. This works best with a void flame though, because void flames can be recoloured by for example putting a blue bubble into the texture. Then it will be blue bubbles coming up. If you have another colour flame it will just be bubbles in the colour the flame itself is.
Sorry if you're getting confused by this.