Cannot understand uplevel command in TCL

I am having some problems understanding the use of uplevel in TCL. I am reading Brent Welch's Practical programming in TCL and Tk and there is an example in uplevel that I cannot understand. Here it is:

proc lassign {valueList args} {
  if {[llength $args] == 0} {
    error "wrong # args:lassign list varname ?varname...?"
  }
  if {[llength $valueList] == 0} {
    #Ensure one trip through the foreach loop
    set valueList [List {}]
  }
  uplevel 1 [list foreach $args $valueList {break}]
  return [lrange $valueList [llength $args] end]
}

Can someone please explain it to me? The explanation in the book does not help me enough :(