Your Keep Rate is a measurement of the jobs you claim without cancelling. It's displayed as a percentage, so for example, if you claimed 10 jobs, but cancelled 4 of them, you'd have a 60% Keep Rate. A customer cancellation will never impact your Keep Rate. It isn't calculated until you've claimed at least 10 jobs, and it only takes into account your past 50 claims (unless you haven't claimed 50 jobs, in which case it counts all of your lifetime jobs).
This number is important, because maintaining a high Keep Rate (above 85%) will give you access to jobs further in the future.
Furthermore, claiming a job without the intention of completing it ends up hurting the other professionals using the platform who might've wanted to work that job, and it inconveniences the customers who end up dealing with last minute cancellations. You can help maintain a high Keep Rate by only claiming the jobs you intend to complete.