We provide the geometrical interpretation for the Becchi-Rouet-Stora-Tyutin (BRST) and anti-BRST symmetry invariance of the Lagrangian density of a four (3 + 1)-dimensional (4D) interacting U(1) gauge theory within the framework of superfield approach to BRST formalism. This interacting theory, where there is an explicit coupling between the U(1) gauge field and matter (Dirac) fields, is considered on a (4, 2)-dimensional supermanifold parametrized by the four spacetime variables x µ (µ = 0, 1, 2, 3) and a pair of Grassmannian variables θ andθ (with θ 2 =θ 2 = 0, θθ +θθ = 0). We express the Lagrangian density and (anti-)BRST charges in the language of the superfields and show that (i) the (anti-)BRST invariance of the 4D Lagrangian density is equivalent to the translation of the super Lagrangian density along the Grassmannian direction(s) (θ and/orθ) of the (4, 2)-dimensional supermanifold such that the outcome of the above translation(s) is zero, and (ii) the anticommutativity and nilpotency of the (anti-)BRST charges are the automatic consequences of our superfield formulation. PACS numbers: 11.15.-q, 12.20.-m, 03.70.+k Keywords: QED with Dirac fields, nilpotent (anti-)BRST symmetry invariance, superfield formalism, horizontality condition, gauge invariant condition on the matter superfields, geometrical interpretation(s)The usual superfield approach [1-8] to BRST formalism has been very successfully applied to the case of 4D (non-)Abelian 1-form (A (1) = dx µ A µ ) gauge theories. In this approach, one constructs a super curvature 2-form F (2) =dà (1) + ià (1) ∧à (1) by exploiting the Maurer-Cartan equation in the language of the super 1-form gauge connectionà (1) and the super exterior derivatived = dZ M ∂ M ≡ dx µ ∂ µ + dθ∂ θ + dθ∂θ (withd 2 = 0) that are defined on a (4, 2)-dimensional supermanifold, parametrized by the superspace variables Z M = (x µ , θ,θ) (with the super derivatives ∂ M = (∂ µ , ∂ θ , ∂θ)).The above super curvature is subsequently equated to the ordinary 2form curvature F (2) = dA (1) + iA (1) ∧ A (1) (with d = dx µ ∂ µ and A (1) = dx µ A µ ) defined on the 4D ordinary flat Minkowskian spacetime manifold that is parametrized by the ordinary spacetime variable x µ (µ = 0, 1, 2, 3). This restriction, popularly known as the horizontality condition, leads to the derivation of the nilpotent (anti-)BRST symmetry transformations for the gauge and (anti-)ghost fields of the 4D (non-)Abelian 1-form gauge theories.The key reasons behind the emergence of the nilpotent (anti-)BRST symmetry transformations for the gauge and (anti-)ghost fields, due to the above horizontality condition 1 (HC), are (i) the nilpotency of the (super) exterior derivatives (d)d which play very important roles in the above HC, and (ii) the super 1-form connectionà (1) = dZ Mà M involves the vector super-fieldà M that consists of the multiplet superfields (B µ , F ,F) which are nothing but the generalizations of the gauge and (anti-)ghost fields (A µ , C,C). The latter are the basic fields of the 4D (non-)Abelian 1-form gaug...