What is meant by the term "gauge singlet"?

Can anybody please explain what is meant by the term "gauge singlet"?
To be more specific, I got the term in a discussion on wess-zumino lagrangian where the superpotential contains the term:$$a_i \phi_i$$. The author claims that in order the theory to be gauge invariant and invariant under susy, the fields present in this term must be gauge singlets. I do not understand this at all. (The fields mentioned are left handed chiral superfields.)

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 Singlet means that something is inaffacted by a certain symmetry transformation. Examples: The neutrino is a U(1) - electric charge singlet. The electron is a SU(3) - color singlet The Higgs boson is a Lorentz: SO(3,1) - singlet (scalar particle)
 Recognitions: Science Advisor In other worlds, "neutral"; zero charge.
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