Standby seat could be few things.
1 there may not be enough economy seats
2 there may be economy plus seats which the gate agent will assign you by the time you arrive to the gate
3 the flight has been oversold and airline is willing to look for volunteers who would take a certain amount as credit for future flight and thus allowing you to be able to go
4 the standby seat sometimes does not get resolved by volunteers and the airline can ask you to be the volunteer and issue a credit voucher for future travel OR make you an involuntarily denied boarding passenger and issue a check in the amount of twice the amount you paid to travel to your missed destination but not more than $650 if you arrive within 1 to 2 hour of your missed flight and four times of your fare but no more then $1300 if you arrive after 2 hours of your missed flight for domestic trip. The airline will still take you to your final destination if available the same day or the next day and issue meal vouchers and hotel as required. Getting a Check is better in most cases unless your voluntarily denied boarding compensation is larger