Under pay-as-you-go rules adopted by Democratic majorities in the House and Senate in 2007, tax cuts or increases in entitlement spending must be offset by tax increases or entitlement cuts. Entitlements include big health programs like Medicare and Medicaid, for which spending is on autopilot, as well as some other programs for veterans and low-income Americans. (Discretionary spending, which includes defense, is approved separately by Congress annually.)
The new Republican rules will gut pay-as-you-go because they require offsets only for entitlement increases, not for tax cuts. In effect, the new rules will codify the Republican fantasy that tax cuts do not deepen the deficit.
I think I'd admire the Republican rules if they required that a tax cut be accompanied by an equivalent reduction in spending. I wouldn't like it, but I'd respect it. As it stands, the rules let the GOP cut taxes to their heart's content without making sure those cuts don't deepen the deficit somehow.
We'll see how this plays out. But there's no evidence to support the idea that Republicans are fiscally responsible. At all.