The code is too complex, as calculated using a well-defined, quantitative measure.
This issue makes it more difficult to understand and/or maintain the product, which indirectly affects security by making it more difficult or time-consuming to find and/or fix vulnerabilities. It also might make it easier to introduce vulnerabilities.
This issue can make the product perform more slowly. If the relevant code is reachable by an attacker, then this performance problem might introduce a vulnerability.
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)