NPK is the formula makeup of plant food (a.k.a. fertilizers) presented as three numbers on the label. The first number represents the amount of nitrogen (N), the second number represents the amount of phosphate (P) and the third number represents the amount of potassium (K). Nitrogen is needed for leafy growth and green color, phosphate promotes root health and potassium helps with overall plant vigor. A good rule of thumb for general feeding of most houseplants is to use a balanced fertilizer like 10-10-10.
Some plants benefit from a higher number of one nutrient than the others. A higher middle number (phosphate) is used to help flowers last longer on plants like African violets and azaleas. Plants that don't flower can use a slightly higher ratio of nitrogen, whereas cacti and succulents don't have a use for excess nitrogen since they don't have green leaves so that number might be lower in food made specifically for them. Again, when in doubt, go for a balanced number like 10-10-10.
Plants only need food when they are actively growing, so spring through fall. Monthly feeding during that time is sufficient, then cut back in mid fall once it starts getting dark by 5pm.