An add-on is a separately installed component that enhances or modifies Confluence. Some add-ons are shipped with Confluence, others are available for you to install yourself. An add-on that specifically plugs into the architecture of an Atlassian application such as Confluence is sometimes called a plugin, although the terms 'plugin' and 'add-on' are often used interchangeably.
A macro allows a developer to perform programmatic functions within a page, and gives the Confluence user access to more complex content structures. Many macros are made available by plugins.
You need System Administrator permissions in order to install and configure plugins.