Lit.: ascent, also spelled aliyah.
These days this expression means the process of immigrating to Israel. Historically it meant Jews coming to live in the Holy Land and thus ascending spiritually.
A modern opposite, yeridah, lit. descent, has sprung up since about the 70's, and accurately denotes the surruptitiously (and sometimes not so...) antagonistic attitude Israelis are prone to have towards those of their numbers who choose to leave and seek their fortunes abroad.