A feature new to 2.9.
Normally, a changing to a different brush preset will change all brush settings.
Locked presets are a way for you to prevent Krita from changing all settings.
So, if you want to have the texture be that same over all brushes, you lock the texture parameter. That way, all brush-preset you select will now share the same texture!
!!!Locking a brush parameter
To lock an option, right-click the little chain icon next to the parameter name, and set it to 'Lock'.
It will now be highlighted to show it's locked:
And on the canvas, it will show that the texture-option is locked.
!!!Unlocking a brush parameter
To unlock, mouse_rightclick the chain-icon again.
There will be two options: