If my taxable income is $5000, why is my tax showing $0? Should it not be 10% or $500?

Any capital gains or qualified dividends?

yes

If your total income was less than $38,600 and your qualified dividends were $5,000 (or whatever the exact number for your taxable income was) then you would be taxed at a 0% rate.

Qualified dividends are removed from your taxable income and taxed at their own rate, then whatever is left over is taxed at your regular rate.