When we write a page of text, that page is typically made up of several paragraphs of text. Each of those paragraphs is made up of several sentences, and finally each of those sentences is made up of words. Likewise when we code, depending on the language we might have blocks of code, chunks of code wrapped in parentheses, etc.
Each of these nouns represents a logical grouping (or object) of text, and Neovim understands (more or less) what each of them are, providing a "higher-level" abstraction that provides some pretty powerful capabilities.
Our goal for the moment is to simply introduce text objects and provide some examples, then we will get into more details about how to work with them shortly.