red line on hit error bar

[walnut]

what do the red lines on the hit error bar mean

[McKay]

There are two kinds of red lines:

Long red lines:
These are called "mis-aims", meaning you clicked but your cursor was not inside the circle (or you clicked too many times unnecessarily). It's useful for checking if you were still within rhythm even while failing to aim correctly, or if you are clicking too much.
You can disable misaims with this ConVar:
osu_hiterrorbar_misaims 0

Short red lines:
These are just "regular" misses and occur very rarely, but can happen if e.g. your cursor is correctly inside the circle but you clicked way too early (inside the miss timing window, which is +-400 ms always outside of the 50/100/300 timing windows which depend on OD).
You can disable misses with this ConVar:
osu_hiterrorbar_misses 0

Use the in-game console to enter ConVars, or persist them in your /<Steam>/steamapps/common/McOsu/cfg/osu.cfg