An envelope in mathematics is kind of like a cover for a letter or a present. It is a special shape that wraps around a family of curves, like a mommy hug for all the curves.
Think of it like this: if you draw a bunch of different curves, like circles, triangles or squiggly lines, and then you put a piece of paper on top of all of them and trace along the edges. Now, if you take away all the lines and look at the outline that you traced, that is the envelope.
The envelope shows you the maximum or minimum points that the family of curves can reach. It's like the best possible picture that you can take of all the curves at once. It helps mathematicians figure out the properties of the family of curves, like their maximum or minimum values, and it can also be used in engineering to find the shape of a container or structure that will fit around a variety of different objects.
So basically, an envelope in math is like a special cover that gives you information about a whole bunch of curves all at once. Cool, huh?