At the risk of beating a dead horse:
Racemic mixture has a specific meaning - it is a 1:1 mix of two enantiomers. While a racemic mixture may, in some contexts, be represented by a wavy bond, the use of the wavy bond is far more general than that single meaning and gives no information whatsoever (in itself) on the proportions of any possible mixture.
The wavy line bond is used in many different ways, depending on context. In the context shown, it means what Babcock_Hall said - the stereochemistry is not specified. The name "glyceraldehyde" with no other descriptors does not show chirality, and the wavy bond in the structure makes that obvious.
It can also mean "unknown chirality" as Discodermolide said. This is usually in the context of a reaction, and I've seen it most frequently in reactions that add a second chiral center to a chiral molecule. This produces diastereomers, and you frequently see some stereocontrol over the second chiral center by the first chiral center. The products are frequently separable by chromatography, but it can take considerable work to determine what the exact structures of the two resulting compounds are. Frequently, both the mixture of diastereomers and the individual diastereomers are represented with a wavy bond until the actual stereochemistry can be determined.
Finally, it is used in some contexts to indicate "not determined" or "not important". This is the case frequently in natural products which have multiple stereocenters, such as steroids. A reaction may produce a new stereocenter which is removed by a subsequent reaction - for example, a hydrolysis reaction to produce an alcohol on a new chiral center, followed by oxidation to the ketone. Steroids are rigid systems and many reactions are very stereoselective, so the hydrolysis reaction will probably yield mostly a single enantiomer - but if you are getting rid of the stereocenter anyway in the next reaction, it probably isn't worth the effort to prove that and determine the exact stereochemistry. Instead, you just represent the C-OH bond with a wavy line, and go on to the next reaction.
There - dead horse is thoroughly flogged.
