While all organisms need a wide variety of nutrients to live, there are a few that are particularly limiting for plants. A limiting nutrient is one that the plant needs to live, but the supply is less than the amount the plant could use, so the growth of the plant is “limited” by the amount of that particular nutrient.
For land plants, two nutrients are often limiting, nitrogen and phosphorous (that’s why these are the two most common nutrients in plant fertilizers!). But nitrogen and phosphorous can come in many forms, different molecules, some of which can be taken up by plants and some that cannot. For example, nitrogen and phosphorous are common components in proteins and enzymes of plants and animals. But a plant root cannot absorb proteins directly. Nitrogen is taken up by plants in the forms of NO3 = nitrate and NH4 = ammonium. Phosphorous is taken up by plants in the form of PO4 = phosphate. Only nitrogen and phosphorous in these forms are “available nutrients” to plants. So only a portion of the “total nitrogen” or “total phosphorous” found in the soil is actually “available” at any given time.