I need help with lua Vocab. (what? and why?)

:IsA is a method of all instances (ROBLOX objects -- parts, scripts, models, services, etc).

It returns true if the object is the type of thing you ask for.

Usually, that will just be checking its ClassName (e.g., a Part has a "Part" .ClassName property).

Here's a simple example that checks if something can be assigned a random brick color using :IsA:

for _, object in pairs(workspace:GetChildren()) do
    if object:IsA("BasePart") then
        -- object will have a BrickColor, since it's a part-like-object
        object.BrickColor = BrickColor.random()
    end
end

ROBLOX organizes the types of objects into a hierarchy -- for instance, WedgeParts, Parts, SpawnLocations, etc, are all BaseParts. You can use :IsA("BasePart") to detect any type of part (where as :IsA("Part") or .ClassName == "Part" detect only Part objects themselves)