Simple way to put it: Being a single battery mod, a mod without boost can only output a max 4.2v to the atty when the battery is at full charge and less when not at full charge. This is because the mod has no circuitry to boost whatever the battery can put out at a given point in time. A mod with boost can output above what the battery's given voltage is at a point in time. It boosts the voltage.
So if you need to be able to vape at, say, 4.0v and above, then you don't want a mod without boost circuitry. But if you are always well under 4.0v, then it may not matter to you.
How to know if a mod has boost or not? Look at the specs. If the output voltage is shown as a max of 4.2v (or less) for a single batt mod, or a max of 8.4v (or less) for a dual batt mod, then you know the mod does not have boost circuitry. It seems most mods and pod mods being manufactured today have no boost circuitry. This trend seems to have started in 2020, maybe late 2019.
For a mod that was introduced to the market in 2020 or maybe even mid to late 2019, if the specs don't mention output voltage at all, then you'll have to do some research and ask around. At this point, it's safer to assume it doesn't have boost than to assume it does because it's a growing trend.
No idea what you mean.