Seriously, if you don't even understand what
srcdir/configure [options] [target] is supposed to do, you shouldn't compile GCC yourself. But as you asked for it...
srcdir/configure is the Linux command for starting the file
srcdir/configure from the current directory (not from "srcdir"!). As of the installation guide, the current directory is "objdir", i.e. the directory in which you want to build GCC. From the installation guide:
We use srcdir to refer to the toplevel source directory for GCC; we use objdir to refer to the toplevel build/object directory. [...] we highly recommend that GCC be built into a separate directory from the sources which does not reside within the source tree.
So, "srcdir" must be the path where you have extracted the GCC source code, e.g. "/home/username/Desktop/gcc-4.5.1", and "objdir" must be a path where you want to build GCC, e.g. "/tmp/gccbuild". When you created the latter, you can go on with the build configuration, so in a terminal, change to the build directory and type "srcdir/configure" (replace "srcdir" with the source code path!!). This should configure the build with standard settings and your current architecture (most probably x86 or x64).
For extended configuration, you must replace the [options] placeholder with the configuration options for gcc, for example
--with-gmp=/usr/local and other settings that are listed in the autoconf scripts. They can also be found in the installation guide under the heading "Distributor options". The [target] placeholder is not necessary as the configure script automatically detects your architecture (that's what the documentation says).
As I said before, don't build GCC yourself except if you really have to. It seems like Ubuntu Maverick will have GCC 4.5 in the near future, so you can grab the Ubuntu package then.