Blocking or bridging can be used for a variety of purposes in deck building.  Probably the most commonly used technique is to install small pieces of material in between the perimeter joists to create a rigid rim joist that prevents bounce and increases the strength of the rail attachment.  Some deck builders install blocking throughout the frame to increase the strength of long joist spans to promote a more solid structure.  

Often times scrap materials from your framing lumber may be used for blocking.  Always make sure that this material is either pressure treated or naturally resistant to decay. 

Another blocking technique involves laying the blocking level with the tops of the joists to create a solid surface around the perimeter to support a picture frame or racetrack perimeter board.  Blocking can also be used in this manner down the center of a deck to provide support for a division board, a zipper pattern, or herring bone pattern.  Most creative decking styles like parquet etc. will require some blocking.