In this excerpt from the book Hymns to Shiva, Utpaladeva's Shivastotravali, Swami Lakshmanjoo reveals who Lord Shiva is meditating on.
Chapter 3 (35:56)
मत्परं नास्ति तत्रापि जापकोऽस्मि तदैक्यतः ।
तत्त्वेन जप इत्यक्षमालया दिशसि क्वचित् ॥१७॥
matparaṁ nāsti tatrāpi jāpako’smi tadaikyataḥ /
tattvena japa ityakṣamālayā diśasi kvacit //17//
O Lord, there is one painting of Your form available in the market.
You have seen that [image] where the Lord is seated with crossed legs, eyes closed, with beads, moving beads [i.e., performing japa], and eyes closed, and thinking of something else. There is one [image of] Lord Śiva in samādhi, in the posture of samādhi, with eyes closed and thinking of something other, higher than His being. [Lord Śiva appears to be] worshiping some other deity.
But, [Utpaladeva] says here, “matparaṁ nāsti”. By that akṣamālā, by those beads, moving those beads in that kvacit, in some photograph of Yours, in some paintings of Yours, You reveal to us, You make us understand, that “matparaṁ nāsti, there is no other higher being than Me (matparaṁ nāsti), but still I recite the mantra of something else”.
So, the recitation of mantra is not reciting a mantra for other lords. You must recite the mantra for your own Self. [Lord Śiva is saying], “I am reciting the mantra for my own nature. I am not diverted towards [some other] higher being. There is no higher being because matparaṁ nāsti, there is no other higher being than Me. Tatrāpi jāpako’smi, even then, I recite a mantra”.
So, in fact, the recitation of mantra means reciting mantra to reveal your own Self.
Tattvena [japa] iti akṣamālayā diśasi. This You teach in that photograph, in that photograph of Yours, that you must recite mantra for your own Self; you must recognize your own nature. You must not go astray to all other devatās (gods).