A custom module is a reusable block you write yourself, in HTML with optional CSS and JavaScript. Once saved it becomes a Custom Module element you can drop onto any page in the builder, as many times as you like. Edit the module once and every page using it updates.
Creating one
Go to Custom Modules in the sidebar. Each module has a name, a description, and its HTML, CSS and JS. There's a starter library of ready-made modules to browse and adapt, with a live preview.
Data sources
A module can optionally be fed live data from your site. Choose a data source of Gigs or Projects and your markup gets that content to render however you want, such as a custom tour-date layout or a portfolio grid that doesn't look like the built-in one. Leave it on None for a purely static block.
When to reach for one
Most things are quicker with the standard elements. Custom modules earn their place when you need a specific layout the built-in blocks don't offer, or when you want the same hand-built block in several places without maintaining copies.
A caution
Custom modules run on your public site exactly as written, so a broken tag or a JavaScript error will show up for visitors. Preview before you publish, and keep third-party embeds to sources you trust.