"Naive implementation" is almost always synonymous with "brute-force implementation". Naive implementations are often intuitive and the first to come to mind, but are also often O(n^2) or worse, thus taking too long too be practical for large inputs.
Programming competitions are full of problems where the naive implementation will fail to run in an acceptable amount of time, and the heart of the problem is coming up with an improved algorithm that is generally much less obvious but runs much more quickly.
