I couldn't sit through that video. Some good high level information but the technical details weren't all correct, and he took forever.
Paint is generally described as 4 ingredients. The three primary are pigment, binder, and solvent. The fourth ingredient is really a category, called "additives".
Pigment is what provides the color. If you remove pigment you have clear paint, also known as varnish.
Binder is also known as medium; it is what cures into a continuous film that is microscopically a matrix of binder molecules with pigment particles trapped in between. For acrylic paint the binder is also known as acrylic medium and sometimes by other more chemical names like acrylic polymer resin.
Solvent thins the pigment/binder mixture so the paint is more liquid and easier to apply. For acrylic paint, the solvent is usually water.
Additives alter the property of the paint. There are additives to thicken, to reduce surface tension, to slow or quicken drying time, to increase opacity, to provide texture, matting agent to reduce gloss, and even more than that.
You can thin paint by adding solvent (e.g. adding water to acrylic paint) but you can add so much that the binder is too thin to cure into a continuous film; this formulation is known as a wash. So beyond a small proportion you're better off adding acrylic medium along with the water.
This is what Thinning Medium is: binder + solvent. If your paint manufacturer offers it, it's formulated to work well with their paints. Many painters mix their own with acrylic medium + solvent, sometimes with surfactant added. Airbrush medium is basically this mixture. Some painters substitute matte medium, which is acrylic medium with a matting agent to reduce gloss.
Surfactant and Flow Improver are the same thing: additives that reduce the surface tension of the paint mixture. This is helpful so paint flows into crevices on models and also so you can turn a wash into a continuous transparent color, known as a glaze or stain. Generally these are added to solvent and the mixture of solvent + flow improver is used to thin the paint. Those sold by the paint manufacturer are formulated to mix well with their paints; many painters though use water with household agents added, like dish soap, dishwasher rinse aid, alcohol or window cleaner (which is water + soap + varying amounts of alcohol and/or ammonia – these reduce surface tension but are there as accelerants).
Retarder is an additive that slows drying time, for wet blending.
