main sweet served with thick tea
In the tea ceremony, the sweet eaten with koicha (thick tea), as opposed to the lighter sweet served with usucha (thin tea).
See also: 濃茶
濃茶の前に主菓子をいただきます。
We have the main sweet before the thick tea.
From 主 (omo, 'main') + 菓子 (kashi, 'sweet'). The reading おもがし is a standard compound reading.