In one sentence
Yoga is both the goal (union with the deepest reality) and the method (the disciplined path by which the union is realised).
The depth — what it actually means
The Gita is itself called a yoga-shastra — a scripture of yoga. Each of its 18 chapters is called a yoga. The Gita names several yogas explicitly: karma yoga (the yoga of action), jnana yoga (the yoga of knowledge), bhakti yoga (the yoga of devotion), and dhyana yoga (the yoga of meditation). It treats them as different roads up the same mountain, suited to different temperaments. The modern association of yoga with physical postures is a derivative — the asana practice is one limb of one yoga (the raja yoga of Patanjali), not the whole.
Modern application — how to use this today
Identify your natural yoga. Are you most alive in action (karma), in understanding (jnana), in love and devotion (bhakti), or in stillness (dhyana)? Most people lean strongly on one. The Gita's instruction is to start where you actually are, not where you wish you were.