What does filing 6 mean

The allowances on the W4 determine how much federal income taxes are withheld from your pay. They correspond with withholding tables used by a payroll department. The higher the number, the less taken out. 6 is a very high number to put on a W4, so its not surprising that it led to no federal income taxes being taken out. Most people claim 0 or 1 on this form. Whether you need to lower that or not depends on whether you now owe because of this.

There's no corresponding figure on your actual tax return.

Social Security and Medicare tax are not impacted by a W4. Everyone pays those at the same rates.