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From WiX Wiki at MindCapers

GUIDs

GUID is an acronym for Globally Unique Identifier. A GUID is hashed from a synthesis of identifiers from various pieces of hardware (MAC addresses from network cards, hard disk identifiers, motherboard serial numbers etc) and a timestamp from the system clock. Broadly speaking it describes a point in space and time. The point of GUIDs is that because it is extremely unlikely to get a duplicate machine signature, much less a duplicate machine signature and an identical timestamp right down to the millisecond, you can count on a GUID to be totally unique.

XML

XML is an acronym for eXtensible Markup Language. XML is defined in excruciating detail in countless books and websites. If you don't know what a WFF is in the context of XML then it is time to do some reading on XML and then some reading on XSLT. There is also quite a bit of information on both topics in MSDN.

ICE

ICE is an acronym for Internal Consistency Evaluator.

An installation package is a database describing the various files, registry entries and shortcuts, how they relate and where they must go in your system, as well as (of course) the binary content of those files. Compiling Wix code transforms the XML you wrote into rows in the tables of this database.

It is entirely possible to write instructions that, while legal from the point of view of the Windows Installer database schema, are contradictory, incomplete or improbable from a semantic perspective. To help you find this type of problem, ICEs analyse the meaning of your code: they scan for instructions and values that, while valid in and of themselves, may cause incorrect behavior in the context of the whole database.

The ICE mechanism is extensible but this is outside the scope of using Wix. Microsoft defines a great number of ICEs.