From the YCM documentation.

> YCM will display a short diagnostic message when you move your cursor
> to the line with the error. You can get a detailed diagnostic message
> with the `<leader>d` key mapping (can be changed in the options) YCM
> provides when your cursor is on the line with the diagnostic.
> 
> You can also see the full diagnostic message for all the diagnostics
> in the current file in Vim's `locationlist`, which can be opened with
> the `:lopen` and `:lclose` commands (make sure you have set let
> `g:ycm_always_populate_location_list = 1` in your vimrc). A good way to
> toggle the display of the locationlist with a single key mapping is
> provided by another (very small) Vim plugin called ListToggle (which
> also makes it possible to change the height of the `locationlist`
> window), also written by yours truly.