This is something that we are focused on right now. As grooming is not one of the 3 scrum ceremonies it tends to get forgotten.
Backlog grooming involves breaking down the highest priority stories (for the upcoming sprint/s) into smaller stories until they are fine grained and well understood by the team. When Sprint Planning occurs these items are then easy to plan and estimate.
The Scrum Guide mentions that teams can spend up to 10% of their time on grooming the backlog. In practice, most of our teams are not close to that 10%.
Some teams like to have regular twice weekly grooming sessions of around 90minutes, whilst others do some every day after their daily stand up meeting. Either way I think that it is a really essential activity.
In my experience, skimping on grooming results in many extra tasks being added mid-sprint, and stories being de-scoped because the team can't get to them (due to these extra tasks that have been added.)
I would love to hear about other peoples experience with grooming - how often, what percentage of times do your teams use?