1. To make something appear superficially attractive; SYN. window-dress.
2. To put on special clothes to appear particularly appealing and attractive; SYN. fig out, fig up, deck up, gussy up, fancy up, trick up, deck out, trick out, prink, attire, get up, rig out, tog up, tog out, overdress.

When you dress up, you wear very nice clothes, often for a special occasion. When you dress people up, you put very nice clothes on them/ often for a special occasion.