The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. This is termed x86, IA-32, or the i386-architecture, depending on context.
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets... After launching the architecture under the "x86-64" name, AMD renamed it AMD64... x86-64 is still used by many in the industry as a vendor-neutral term, while others, notably Sun Microsystems (now Oracle Corporation) and Microsoft, use x64.
So Ubuntu ISO support both the flavors.
Multiarch lets you install library packages from multiple architectures on the same machine. This is useful in various ways, but the most common is installing both 64 and 32-bit software on the same machine and having dependencies correctly resolved automatically. In general, you can have libraries of more than one architecture installed together and applications from one architecture or another installed as alternatives. Note that it does not enable multiple architecture versions of applications to be installed simultaneously.
This Computer system has 64-bit Kernel Architecture:
$ dpkg --print-architecture
Multi-arch support allows you to use 32-bit libraries alongside 64-bit libraries.
This Computer system also supports i386 Architecture (i.e. supports 32-bit Libraries too):
$ dpkg --print-foreign-architectures