You've probably heard the word "integration" thrown around a lot when talking about Zoho, connecting it to your website, your accounting software, or a telephony system. Underneath almost every integration is something called an API, and once you understand what that actually is, a lot of Zoho conversations suddenly make a lot more sense.
API stands for Application Programming Interface. Think of it as a waiter in a restaurant. You, the customer, don't walk into the kitchen and cook your own meal, you tell the waiter what you want, the waiter takes that request to the kitchen, and brings back exactly what you asked for. An API works the same way between two pieces of software: one system asks for something in a very specific, agreed format, and the other system hands back the answer, without either system needing to know how the other one works internally.
Every time Zoho "talks" to another system, whether that's pulling orders in from Shopify, pushing invoices out to accounting software, or updating a record because someone filled in a form on your website, an API is doing the work behind the scenes. When someone refers to "the API" in a Zoho conversation, they usually just mean the specific set of rules Zoho (or the other system) has published for how these requests can be made.