This feature is quite straightforward and the title indeed does it justice. Version 1.14.17 increases the transaction account lock limit from 64 to 128. When a transaction on Solana is composed, it must specify all the writable accounts it wishes to access. These writable accounts require locks to prevent race conditions where multiple parties are trying to read and write to the same accounts which can result in incorrect state being returned to the caller. As such, writable accounts are locked. While an account is locked however, no other transactions that need to write to the same account can be executed. Thus, the more accounts a transaction locks, the other less transactions writing to that account can be parallelized. With this in mind, the increased account lock limits should be used judiciously so as to not unnecessarily consume excess cluster resources.
Note that this feature is gated as explained in the v1.14.17 Release Summary. Therefore, it is included in the release but will not go live till the feature gate is activated. You can track activation of the feature through the associated GitHub issue.