Orchids have a reputation for being difficult to care for. Some species are. But most will respond well if you remember the old saying 'feed weakly, weekly.' Fine advice, but feed what?
Like all plants, orchids thrive on a combination of nitrogen (N), phosphorus (P) and potassium (K). Some of that they get from the growing medium and air. But potted plants (which cover most orchid plantings) need supplements. That's especially true since orchids are not potted in soil but bark, rocks, sand or some other appropriate medium.
Those supplements come in a variety of forms: pellets, liquid, mulch and others. But whichever form suits your convenience the essential fact is the ratio of these three elements.
When you see a bottle or package of fertilizer, they will often be labeled 30-10-10 or 15-5-5 or even 20-20-20. These are the relative amounts (in percentage terms) of the three ingredients. Observe that they don't add up to 100%. That's normal. The remainder is composed of inactive ingredients - sometimes water, sometimes an inert material to hold the substances together.
For orchids growing in the barks of house trees, a common decorative method that emulates the behavior of some species (epiphytes) in the wild, the stronger mixture (30-10-10) is desirable. Potted orchids will do well with an even amount (20-20-20). But even those ratios should be altered at different times of the year.
When the Summer fades and you want to give a final boost to your orchid blooms a 10-30-20 mixture is a good idea. The early Spring, before the first bloom, is also a good time to provide a little more of those needed elements. That produces full, healthy flowers.
But take care not to overdo it. Orchids are sensitive plants for the most part and fertilizer burn is common for overfed samples. To avoid that problem, fertilize once per month at full strength, then dilute the mixture to one-quarter strength for the other weekly feedings. Also once per month, simply water without feeding at all to rinse out any accumulated salts in the soil.
Potassium and other elements combine readily with elements or molecules in the soil (Chlorine, CO2, ...) to form salts that alter water absorption, change biochemical reactions and have other effects. A small amount won't kill the plant. But everything in gardening is a matter of degrees. Too much is harmful.
Sometimes fertilizer mixtures will list 'potash' on the label. This is nothing more than a traditional word for potassium, since that compound was a common source of the element for generations.
Since it can be leached out of the soil very easily, owing to its ready ability to dissolve in water, it's helpful to provide it in multiple forms. Liquid potash comes in sprays that can be applied directly to the leaves or sprayed onto the soil.
In general, it's better to apply too little than too much fertilizer. But if you follow the directions that came with your species you will often get it just right. Regularity is key.