A single battery has 4.2 volts.

If you fire a 0.8 ohm coil at 20 watts it will need 4.0 volts.

Same coil, you vape on it at 35 watts, it will need 5.29 volts.

But, wait, if the battery has only 4.2 volts, how can we achieve 5.29 volts?

This is where boost chip enters the scene. It allows you to reach higher volts. If your mod has no boost it will still fire, but will lack in power because is limited to battery output.

By looking at the specs you can know if a mod has boost.

If voltage range is not stated in the specs, the mod probably has no boost.