What the table is
The Base Ignition Angle table is the ECU's main timing reference, the number it starts from before any ignition corrections for temperature, gear, knock and so on are applied. If the base table is wrong, every correction is working from a bad starting point, so this is where timing work begins.
Axes
The table is designed with Ignition Load across the top, based on either MAP or TPS, and RPM up the side. To change the breakpoints, select the table and press F3, then edit an existing value or add a new one. When you move an existing breakpoint, NSP rescales the cells so the table keeps its shape, so you can refine axis spacing without redrawing the map.
Reading the numbers
Each cell is the requested timing in degrees before top dead center (BTDC). A negative value asks for timing after top dead center, which you'd only expect in special cases such as some anti-lag or heavy-retard strategies.
Add timing in small steps under load, watch for knock and confirm torque gains on a dyno. More timing isn't always more power. See ignition timing vs air-fuel ratio.
Closed throttle timing lives in a separate table: ignition zero demand. Cranking timing has its own table too, see ignition cranking.
