Crawling code is the practice of scanning a code base of the review target in question. It is in effect looking for key pointers wherein possible security vulnerability might reside. Certain API's are related to interfacing to the external world or file IO or user management which are key areas for an attacker to focus on. In crawling code we look for API relating to these areas. We also need to look for business logic areas which may cause security issues but generally these are bespoke methods which have bespoke names and can not be detected directly, even though we may touch on certain methods due to their relationship with a certain key API.
Also we need to look for common issues relating to a specific language. Issues that may not be *security* related but which may affect the stability/availability of the application in the case of extraordinary circumstances. Other issues when performing a code review are areas such a simple copyright notice in order to protect ones intellectual property.
Crawling code can be done manually or in an automated fashion using automated tools. Tools as simple as grep or wingrep can be used. Other tools are available which would search for key words relating to a specific programming language.