The predictions of the Standard Model agree very well with the results of experiments that investigate properties of elementary particles. It is because of this that the scientists are sure that the Standard Model, at least parts of it, is a good theory for elementary particle physics.
However, the Standard model is not free from flaws. One example of this is that the Standard Model, in its simplest form, predicts all particles to be massless. This is obviously not correct; all leptons and vector bosons have mass that has been measured in experiments.
To solve this problem, the so-called Higgs mechanism is introduced. This mechanism is a mathematical operation that is applied to the equations of the Standard Model. By introducing the Higgs mechanism, the equations of the Standard Model are changed in such a way that the vector bosons become massive. After this change there is also no problem that the leptons have mass.