MongoDB employs a locking system to allow multiple clients to read and write at the same time, ensuring data consistency. Prior to version 2.2, MongoDB used a single reader/writer lock per instance, but since then, concurrency has improved with more granular database-level locks and long-running operations yielding their lock under certain conditions. Lock contention can significantly impact performance, causing issues such as slow application calls, timeouts, and failures. A healthy system typically sees a low lock percentage, averaging around 80% or lower, which may vary greatly depending on the use case. To understand normal lock percentages for a specific database, it's essential to examine trends over time using MongoDB Management Service (MMS) and establish a baseline to track any significant deviations from that threshold.