Bootstrappers are programs that run a sequence of installation packages one after another. This is often needed when a product has to have certain prerequisites installed.
Windows Installer does not support "nested" installations, where one package starts its installation, pauses in the middle to run a separate "inner" installation, then picks up the "outer" installation where it left off. Instead, you want to run a chain of installations in sequence, one by one.
A program that does this (not an installation itself, but a runner of installations) is called a "chainer" or a "bootstrapper". Bootstrappers are often smart enough to install one version of a package instead of another, or to not install a particular package at all, depending upon conditions on the target machine.
Why you might need this: a product (main install package) that requires the Visual C++ runtime libraries (prerequisite package #1) and the .NET Framework (prerequisite package #2) to run. The bootstrapper can lay down the prerequisites first (or skip them if already installed) then lay down the main product.
WiX burn is a bootstrapper.
