Or the concentration on subtle sense perceptions can cause steadiness of mind.
The inner concentration on the process of sensory experiencing, done in a way that leads towards higher, subtle sense perception; this also leads to stability and tranquility of the mind.
In these sutras, Patanjali is giving some examples of meditation practices. The last sutra mentioned focusing on the breath, this one is simply focusing on the senses.
Again, this is in contrast to the Zen practice of zazen, or ‘just sitting’. We do not focus our attention on anything in particular, but are mindful of our breathing going on, the feeling of our body on the cushion on the floor, the sounds coming to our ears. We notice this coming and going without judgement, but without the specific focus that is being offered here.