What information is encoded in an American passport's numeric code along the bottom of the front information page?

From https://www.sinosecu.com.cn/en/news/view/20.html (mirror):

The International Civil Aviation Organization (ICAO) is responsible for setting up standards for Machine Readable Travel Documents (MRTD). MRTD makes it easy for automated travel documents checking. It’s much faster and less tends to make mistakes than manual input.

MRTD have a Machine Readable Zone (MRZ) at the bottom. These MRZes are printed in the monospace font OCR-B. Passport is included in MRTD. It has two lines and each line has 44 characters. The following picture shows the pattern of a passport:

More information:

• As DetlevCM mentions in their comment, https://en.wikipedia.org/wiki/Machine-readable_passport is worth checking out too.
• ICAO Doc 9303 (mirror), which specifies Machine Readable Travel Documents.

Code:

• https://github.com/konstantint/PassportEye: "PassportEye: Python tools for image processing of identification documents."
• https://github.com/evermeer/PassportScanner: "With PassportScanner: use your camera to scan the MRZ code of a passport on iOS."
• https://github.com/DoubangoTelecom/ultimateMRZ-SDK: "This is state-of-the-art Machine Readable Zone / Travel Documents (MRZ / MRTD) dectector and recognizer using deep learning. The Github repository contains the SDK binaries for Android, iOS, Raspberry Pi and Windows."