making or authorizing an expenditure from, or creating or authorizing an obligation under, any appropriation or fund in excess of the amount available in the appropriation or fund unless authorized by law. 31 U.S.C. § 1341(a)(1)(A).
involving the government in any obligation to pay money before funds have been appropriated for that purpose, unless otherwise allowed by law. 31 U.S.C. § 1341(a)(1)(B).
accepting voluntary services for the United States, or employing personal services not authorized by law, except in cases of emergency involving the safety of human life or the protection of property. 31 U.S.C. § 1342.
making obligations or expenditures in excess of an apportionment or reapportionment, or in excess of the amount permitted by agency regulations. 31 U.S.C. § 1517(a).
This has several practical effects. Most notably, if Congress does not pass the Federal Budget by October 1st, the government must shut down.
This also means that the government generally cannot accept free services. In practice, agencies do accept credits from vendors that they have existing relationships with. Agencies can also use Open Source software - though the House only recently allowed itself to use it. However, staff cannot independently sign up for free websites and services for official use - for instance, task tracking/project management tools should not be used without official permission. (There are also records management and cybersecurity reasons for this as well.)
There are also implications for expectations for employees as well - though the specific details sometimes vary from agency to agency, and are often related to policies negotiated with unions. For instance, some agencies prohibit the use of personal cellphones for generating tokens for multifactor authentication, requiring those agencies to issue token-generating devices to all staff.