This feature allows the client to reset the start date of their listing to now. While that doesn't sound like a lot it can have an impact for sites that have their default display order set to "newest listings first". The software comes installed with "newest listings first" by default when installed. So essentially this allows the client to temporarily "bump" their listings to the top of the category browsing results and newest listings modules until a new listing is placed after them or another client bumps their own listing behind them.
Look to the following setting within the admin tool:
LISTING SETUP > GENERAL SETTINGS > Allow sellers to bump listings to top of category browsing results temporarily
To turn this feature off set that configuration's value to 0. That configuration when not 0 determines the number of days between use for any individual listing. So if you set this configuration to "5" the client will only be able to bump any individual listing every 5 days. So 5 days after they place their listing the bump button would appear for that individual listing within that sellers "my current listings". As this setting is calculated per live listing the client could have all, some or none of their listings available to be bumped at one time.