Welding is joining two metal parts by melting their edges, with or without a filler material, so that they become a single piece.
In a grille, a gate or railings, the welding is what holds the bars to the flat bars and the frame. A solid bar is no use if it is held by a small spot weld that breaks with a single blow.
A properly made weld is continuous and has no cracks or pores. A weak one can be spotted because the bar moves, because there is a crack in the weld bead or because rust is showing right at the joint.
On the coast, the weld is the first place rust appears if it was not properly protected after welding. Check those joints from time to time. If a grille has loose bars, our security grilles page explains how we check it.