This shloka and the next (5.28) together describe the method of meditation. Shut out external sense-objects. Fix the gaze between the eyebrows. Balance the prana and apana — the incoming and outgoing breath — as they flow through the nostrils.
This is the same practice taught in meditation traditions. Draw the mind inward. Leave aside the noise, the distractions, the wandering thoughts. Bring attention to the breath. This is the practical road of sadhana.