Well, normally to get an Airlock you would need to open the sub-menu for it in the build menu and find the part among the list of all other parts.

This mod removes it from the sub-menu's so that Airlocks are always displayed, right next to doors and corridors.

The mod is part of a series of three mods that do the same for different parts. All three mods are also in a combined mod, for ease of use.

The mod series is labelled "Always Essential" because these are parts that every ship should ideally have, or are used often whilst building out other sections. Having them on 'constant' display is also a good reminder that you should be placing them.

I don't understand the purpose of this mod, please explain more.