Base nutrients are those your plant cannot grow without (unlike additives and boosters). Base nutrients generally split into Grow (vegetative) and Bloom (flowering).
One part base nutrients are the simplest to use and recommended for beginners.
Two part nutrients are generally A&B formulations and dilution is varied at different plant stages. Three part nutrients offer the most flexibility to play with nutrient ratios depending on plant conditions.
Base nutrients can be organic or non organic. Generally organic nutrients are best in soil as they can clog hydro systems. There are also special coco, rockwool and aqueous base nutrient solutions. Hard and soft water ranges are also available – if your kettle furs up choose Hard water nutrient ranges.