Assets are design files the developers require to re-create what you designed, using HTML/CSS/JS in a website or native languages like Swift or Java in iOS and Android apps.
In general, the most common assets are bitmap images, icons, and background patterns that you can export in these formats: PNG, SVG, PDF, and JPG.
With XD 57 and later, you can export JPG, PNG, and PDF assets with consistent color renditions. To achieve this, switch the document’s color profile to sRGB. If you try to export an asset from a document that has an unmanaged color profile, you get a message that tells you about a possible color discrepancy in the exported asset. For more information, see Color Management.