A custom font is any non-system-default typeface you manually upload to your website. You can find them on free font websites or font-hosting services like Google Fonts. Custom fonts usually appear in OpenType Font (OTF), Web Open Font Format (WOFF), or TrueType Font (TTF) formats. More formats exist, but these 3 are the most common and compatible with browsers.
In Webflow, go to Site settings > Fonts to upload your fonts of choice. Then you can use them throughout your website to add a unique touch and help your design stand out. Manage your fonts by navigating to Style panel (S) > Typography.