Uttam Kshama means the Supreme Forgiveness. It is the first of the ten Dash Lakshan Dharma in Jainism and is especially observed during the Das Lakshan period and Paryushan. Day 1 refers to the first day of that Das Lakshan emphasis on cultivating forgiveness in thought, speech, and action.

Simple meaning (arth):

• Forgive others for their mistakes and seek forgiveness for our own.
• Keep the mind free from anger, pride, deceit, and greed.

If you want, you can read more about Uttam Kshama Dharma on a Jain Knowledge page here: