The language is similar to forth and is stack based.
Because it is stack based, it ensures that each instruction es exetuted exactly once and, importantly, there are no loops.
That also means that the language is not Turing-complete. (In contrast, Ethereum's scripting language is Turing-complete).
Such scripts are used to determine if a transaction is valid.
Theoretically, it is possible to create scripts that can put a criterion to be met in order for a transaction to be valid. Practically, it turns aut, that most transactions share the same script.
The possible opcodes of the language are defined in sources/script.h.