(used before a disyllabic verb, often after an adverb like 不[bu4], 大[da4], 稍[shao1] etc, to indicate that the action applies to sth previously mentioned, as in 稍加改良[shao1 jia1 gai3 liang2] "make some minor improvements to (it)")
Examples
二加三等于五
Èr jiā sān děngyú wǔ.
Two plus three equals five.
功上加功。
Gōng shàng jiā gōng.
To add merit upon merit; to make further efforts on top of achievements.