Definition
Checking Development Build in the Build Settings defines DEVELOPMENT_BUILD, enables the Profiler connection and script debugging, keeps more logging, and may include debug symbols. The player is larger and slower than a release build, and it allows the Profiler and a script debugger to attach.
Relevance
A development build is for testing on a device. The Profiler can connect, a script debugger can attach, and logs stay verbose. The player is larger and slower than a release build, so the flag stays off for the build you ship.
Example
You make a development build, connect the Profiler, and attach the script debugger. The player hits your breakpoint in TakeDamage, and you read the health field on the device.