Shukr is being grateful to Allah by thanking Him with the tongue, and admitting ones blessings by the tongue.
It’s also with love in the heart for the blessings Allah has given and it’s obedience to Allah with the limbs with acts of worship. This is mentioned by Ibn Qayyim (rh).
There are 5 principles for gratitude:
- Submission of the grateful one to Allah
- Love for Him
- Admitting the blessings
- Praising Him for that
- Not using the blessing in disobedience to Him.
This is why the Prophet (saw) would stand all night in prayer and when asked why he said, should I not be a grateful Slave!