In one sentence
Moksha is the final goal of human life in the Hindu scheme — liberation from the cycle of repeated birth and death, and from the suffering bound up with mistaken identification.
The depth — what it actually means
Moksha is not heaven and not annihilation. It is the realisation that the bondage was a misidentification — the long forgetting that the real "you" was never bound, never small, never separate. The four goals of human life in the classical scheme are dharma (right conduct), artha (material wellbeing), kama (legitimate pleasure), and moksha (liberation). The first three are pursued in the world; moksha is the recognition that lifts you beyond the world while you continue to live in it. The Gita's contribution is to make moksha compatible with full engagement in life — the karma yogi is liberated while still acting.
Modern application — how to use this today
You do not need to believe in literal reincarnation to find moksha useful. Translate it as: liberation from the smaller story of yourself. Every time you stop identifying with a passing thought, a passing role, a passing emotion as if it were the whole of you — that is a small moksha. The full one is the same recognition made permanent.