A still mind is focused, not empty. It is aware, not blank. It sees, but thoughts to not rush in. It does not react. It is engaged.
Think of the times you are happy. When are you joyful. Content. Fulfilled. At peace. In love.
They are likely times when you are concentrated. When your mind is focused on what is at hand. When extraneous thoughts do not distract you. When you are in control.
It might be in nature in a glorious landscape. It could be at a task in which you are fully engaged. It may be in the arms of your lover or in conversation with a mentor. It may be in a concert hall or theater, or an arm chair with a favorite author.
It could be in meditation.