In a password spraying attack, an attacker tries a list of common passwords against a large number of accounts. The attacker does not attempt to authenticate with each account multiple times, but rather tries a single password against a large number of accounts. This type of attack is often used against organizations with a large number of users, as it can be more efficient than trying to brute-force each account individually.
In this case, the security analyst notices multiple authentication failures from different usernames that share the same source IP address. This is a strong indicator that a password spraying attack is taking place.