Plants need different elements from the soil to grow. If they are not present in the right amount the plants will not be as healthy or grow as they should. This is the reason plants need fertilizer, to add the missing elements (nitrogen, phosphorous, and potassium). You can give your plants too much fertilizer and that will have the opposite affect you are going for.
The same species of plant that is grown indoors or outdoors will have different fertilizing needs. Plants that are grown indoors have a will not need as much fertilizer as the ones grown outside. The rate of growth is slower indoors and you can harm your plant by over-fertilizing it.
How do you know how much fertilizer to give to your plants and when to feed it to them? If you have purchased a plant meant for the indoors chances are the information tag it comes with will give fertilizing instructions. But a better method to follow is observing your plant for any signs that would indicate it is lacking nutrients.
Your plant might need fertilizer if it is not growing as fast as it should. If you notice the growth of the plant is stunted and it is spring or summer you can safely add fertilizer in small amounts over a period of one to two weeks. Many indoor gardeners will add fertilizer as part of routine maintenance each spring and that is fine too. Just be careful not too over-fertilize. Some signs that you have added to much fertilizer are the tip of the leaves are turning brown or the leaves are drooping downwards.
All fertilizers are not made the same; they are available in different ratios of nitrogen, phosphorous and potassium. Purchase a fertilizer meant for indoor plants or one that is made for a specific type of plant.