To add up is to make sense as a whole, or to total numbers. It doesn't add up, add up the column, the story adds up.
The figurative sense is a quiet accusation: saying an account does not add up means you suspect it without saying so. The literal sense is arithmetic. It also means to accumulate — the small costs add up.
1phraseadd upTo make sense as a whole, or to total numbers.
“His story doesn't add up; he says he was at home, but his car was seen in town.”
add up is a totalling, or a making sense.