A font is software, and the licence decides what you may do with it. Here is the short version of each type you will meet.
Open licence (OFL, Apache)
Free for personal and commercial work, free to embed in documents and apps, free to modify. Keep the licence file with the fonts. If you modify and redistribute an OFL font, you must not keep its reserved name. This is the safest category for most projects, and it covers most families on Google Fonts.
Free for personal use
Free to download, restricted in use. Client work, products for sale, monetised video and company branding are normally excluded, and the designer usually sells a separate commercial licence. If a download page offers a "commercial licence" button, that is your signal that the free version is personal only.
Desktop licence
Lets you install the font and create artwork. Usually counted per user or per computer. It does not include serving the font from your website.
Web licence
Covers embedding as a webfont in CSS, and is often priced by monthly page views. Self-hosting a desktop-only file for a website is one of the most common breaches - and easy to detect, since the file is served publicly.
App, game and broadcast
Embedding a font inside an app or game, or using it in broadcast material, is typically a separate tier. Ask before you ship: retroactive licences cost more than planned ones.
Subscriptions
Adobe Fonts, Monotype and similar services grant activation rights while you subscribe. Cancel, and the fonts stop being licensed - so long-lived brand assets are safer with a perpetual licence.
System fonts
Fonts that ship with an operating system are licensed for use on it. Reference them in a font stack for web work rather than uploading the file.
Trademark is a separate question
Even with a valid font licence, a brand's logo or title treatment can be protected. Recreating a film logo with a lookalike font does not transfer any rights.
A five-minute check before delivery
- Find the licence file in the download.
- Search it for "commercial", "web" and "embed".
- Confirm the number of users or page views allowed.
- Save the licence with the project archive.
- Email the foundry if anything is ambiguous.
More detail, including how this site labels licences, is on the font licensing page.