Based on the excerpt, the portico is most likely a structure out in the open air and which usually is a roof supported by columns and in this example probably like a large porch wherein the inscriptions could be read by moonlight since it is outside.
Read this excerpt from The Riddle of the Rosetta Stone by James Cross Giblin. "I will not try to describe the impression that the temple, and in particular its portico, made on us," Champollion wrote. "The separate dimensions of the structure can be measured, but it is quite impossible to give an idea of the whole. We stayed there two hours, filled with ecstasy. Led by our native guide, we wandered through the halls and tried to read the inscriptions on the outside in the glittering moonlight." Based on the context clues in the excerpt, a portico is most likely