When something is on the table, it's an option, especially one that could be negotiated. If someone offers you a job but you haven’t said yes or no yet, the offer is still on the table.
When two people negotiate something — like a contract — they often do it over a table. When something is on the table, it's up for discussion. If an employer and a union are willing to discuss pensions, then pensions are on the table. If one side won't budge on something, then that topic is off the table.