Taxable income is the income (revenue) given on tax. It is payable, in other words, it is the tax paid for being paid.
Example:
You get $40
You need to pay 5% to tax
5% of 40
.05*40 = 2
2 is the tax, so,
40 - 2 = 38
Your revenue is 38 now, and taxable income is 2