Why An antenna's radiation pattern is a plot of relative field strength in every direction, and it typically has one dominant peak plus smaller side and rear lobes. The main lobe is that dominant peak, the direction in which the antenna concentrates the most energy, so it is where the radiated field strength is greatest. Gain figures and beamwidth are both measured with reference to this main lobe.
Watch out The choices about maximum current or maximum voltage standing wave points describe conditions along the wire of the element itself, not features of the radiation pattern in space. Vertical angle of radiation describes the elevation of the lobe, not the definition of the lobe.
Lobe = a bulge in the pattern; the main lobe is the biggest bulge, pointing where the signal is strongest.