This phrase has two meanings.
Those who like formal methods are talking about using mathematics and logic to solve problems and develop software. They tend to be quite agile in the way they use one or more formal languages based on discrete mathematics and symbolic logics. The agility comes from the symbols, and from experience in using them. However some formal methods use powerful tools.
On the other hand, others equate formal methods with: too much documentation, high ceremony, and being told what to do. They, therefore, don't like formal methods.
Hmm..then there is yet another (more intelligent) group out there who sees the advantages of both formal methods and pragmatic methods.. combining all aspects of testing, formalizing, and specifying. Any formalist worth his salt should fill in some values and try their specs out. Every math teacher in the world that I knew (and every scientist), always tested his/her equations with experiments, pencil, and paper. See also TheBestOfBothWorlds, WinWin, and ViolentAgreement.