What does reduce: get used for?

I'm not completely following what its purpose is? Below is the code from the Magritte, living in the SequenceableCollection object: reduce: aBlock | result | self isEmpty ifTrue: [ ^ nil ]. result := self first. 2 to: self size do: [ :index | result := aBlock value: result value: (self at: index) ]. ^ result In my case, it is causing problems on VW when it calls aBlock (near the end) and passes in a pair of Characters which are processed by MAComponentRenderer>>classFor: which then tries to do a string append (#,) which gives a DNU since VW's characters (and I think Squeak from what I can tell) do not have concat methods (#,).. Am I missing something? Below is the code that calls reduce: MAComponentRenderer>>classFor: aDescription | classes item | item := (component description = aDescription ifTrue: [ component ] ifFalse: [ self childAt: aDescription ]) ifNotNilDo: [ :comp | comp model ]. classes := aDescription cssClassesForItem: item. (self hasError: aDescription) ifTrue: [ classes add: 'error' ]. ^ classes reduce: [ :a :b | a , ' ' , b ]