Broadly, the German constitution strongly limits the extent to which the budget can involve deficit spending - this is the so -called schwarze Null, the black zero, or the principle that the budget should be basically balanced.
In the pandemic, the coalition used emergency funding which overrode the black zero but this being Germany, managed not to spend all of it, and ended up with a €60bn surplus. The government diverted the surplus into the KTF, the climate and transformation fund, which as it sounds is (was) intended to support the transition to low-carbon technologies.
The argument was that this was against the constitution because it provided an unjustifiable end run around the schwarze Null - a future government could use an emergency to justify going into a larger-than-necessary deficit, and then divert the excess into the budget. The Bundesverfassungsgericht, the constitutional court, agreed and pulled the plug on the scheme, spectacularly screwing up the federal budget for the year.
Obviously this is much duller than the US version of shutting down the government every year in a spate of willy-waving, or the UK version of shamefacedly going deeper into debt while pretending not to.