What does it mean?

"have / keep something up sombody's sleeve' is the complete form of the idiom.

It means 'to keep a plan or an idea secret unitl one needs to use it'.

For example, 'I have a few ideas up my sleeve'.

中文可解作 '在未實行前將計劃保密', 將這idiom形象化, 就能比喻作把東西藏在手袖裡.

Hope this helps!