An example of notelock: the second note is inaccessible
Notelock, or note lock, is an informal term for a gameplay mechanic of osu! which may prevent a player from clearing a hit object. It happens if two conditions are met at the same time:
In this case, the second object is said to be locked behind the first one, which makes osu! ignore the player's input on it until the first object's hit window has passed. Notelock only becomes a problem if the player is unable to return to the previous object and interact with it properly, thus removing the lock. In this case, a locked note may cause a cascading failure, forcing the player to miss more and more subsequent objects, until they run out of health and fail the beatmap.
When notelock occurs, the clicked hit circle will shake. This does not happen for sliders and spinners.
Notelock is a part of osu!'s timing system and happens when the timing windows of two objects overlap. It occurs more often on beatmaps with low OD or high BPM values, because timing windows may overlap more frequently.
Since objects in regular osu! beatmaps are meant to be cleared in chronological order, input rejection caused by notelock usually plays a positive role:
From a mapper's perspective, notelock could be prevented by carefully choosing the OD value of a difficulty according to its object density and BPM. On beatmaps with BPM of 200 and higher, OD 5 and higher is recommended. For a more detailed breakdown, refer to the "Avoiding notelock at high BPM" guide.