What are "bind variables"?

I'm not talking about a search API, I'm talking about code units that ask myriad questions of the database in order to implement myriad business logic. If you don't have the problem of asking myriad questions of the database, fair enough, but that is a simpler database-connected-software use case than any I've come across (including the ones I thought were pretty simple). I'm very curious now what your service does :)