Can someone please explain to me what exactly it means to "seal" the hair?

What is the process? When you do you apply? How much? How often? etc. etc.

I've heard a lot about sealing but I can't seem to find anything on technique. I have a huge bottle of grapeseed oil that I'd like to try out, but I'm not sure how to incorporate it into my routine.

I'm pretty sure I have med-high porosity and fine hair.

Thanks!
Ontario, Canada curly following CG since March 2013 (I'm new!)

Hair Type: 2C/3A
Porosity: High (??)
Density: Medium
Width: Mixed

Trying something new:
Low-Poo: Giovanni 50:50
CO: Suave Naturals Strawberry Conditioner
RO: TN Vibrantly Smooth Conditioner
LI: President's Choice Hydrating Conditioner (protein-rich); Biolage Conditioning Balm
Styling: HETT Curl Gel; AVG; Biotera firm-hold styling gel