C’est une autre paire de manches is a French expression that translates literally to “it’s another pair of sleeves” and means “it’s another story”, “it’s a whole different ballgame” and “it’s another kettle of fish”. A French translation is c’est plus difficile (it’s more complicated).
C’est une autre paire de manches
It’s another story, it’s a whole different ballgame

Example sentence
J’ai monté beaucoup de montagnes mais celle-ci est une autre paire de manches : elle est beaucoup plus haute plus raide.
I’ve climbed a lot of mountains but this one is a whole different ballgame. It’s a lot higher and a lot steeper.
Grammatical explanations
- There is a lot of confusion as to whether it’s j’ai monté la montagne or je suis monté la montagne. The verb avoir (to have) is used as an auxiliary verb in this case rather than être (to be) because monter (to climb) takes the direct object la montagne (the mountain).
- To say “a lot of + plural noun”, you must say beaucoup de + noun and never beaucoup des + noun.
- Celle-ci (literally this one here; feminine) is a compound demonstrative pronoun.
- The adjective raide can translate to both stiff and steep.