Serrés comme des sardines is a colloquial expression. Literally “packed together like sardines”, it means to be squeezed in a very small space. If you’re maybe taking the metro or the train on rush hours, you might not have room to move. Then you are serrés comme des sardines.
The expression appeared at the end of 19th century, at the time the first sardines were canned!
French with Valerie 