Prefixes are given to table names so they cannot be easily guessed by a hacker or malicious user. When guessed, the default database table prefix can make life easy for a hacker and enable attacks (like SQL Injection) to be easier to execute successfully. By using the default prefix, your WordPress database security is at risk of being compromised.
When renaming the table prefix try to avoid using regular dictionary words or names based on user names or common variations for the standard prefix. The more uncommon the prefix, the harder it is to guess. Read this tutorial to change the WordPress table prefix: How can I change the WordPress database table name prefix?