al-Shukr is interpreted in two senses:
1. al-Shukr is used in the sense of tawhid (the oneness of God), i.e., as muwahhid (one who affirms the oneness of God); as in the following verses:
We will surely reward the grateful {that is, the muwahhidūn}. (Āl ʿImrān 3:145)
Is not Allah most knowing of the grateful {that is, the muwahhidūn}? (al-Anʿām 6:53)
And if you are grateful {that is, if you are muwahhidūn}, I will surely increase you. (Ibrāhīm 14:7)
There are many similar verses.
2. al-Shukr is used in the sense of gratitude for a blessing / giving thanks for a favor; as in the following verse:
Be grateful to Me {that is, for My blessings} and do not be ungrateful! (al-Baqara 2:152)
{He (Suleiman) said}: "This is by the favor of my Lord; will I be grateful {that is, grateful for His blessing}, or ungrateful?" (al-Naml 27:40)
(We granted wisdom to Luqman) so that he might be grateful to Allah. Whoever is grateful {that is, grateful for a blessing}, is only grateful for [the benefit of] himself. And whoever is ungrateful—then indeed, Allah is Free of need and Praiseworthy. (Luqmān 31:12)