You have to click on some of the links. Check under docs, you'll find details about several different systems that are all under the umbrella of "plain text accounting". It's not one app. They have links to information on ledger, hledger, and BeanCount which all work similarly but are not (entirely) compatible (in particular, I know there's a common subset of both ledger and hledger which covers most personal accounting needs, but they each have some features the other lacks, never used BeanCount).

https://plaintextaccounting.org/#docs