Possible meanings are 1) "Pay attention to God's instructions and teaching" or 2) "Then you must remember the teaching and testimony I gave." ([Isaiah 8:16](../08/16.md)).
Isaiah speaks of people who do not know God as if they were people walking in darkness without any light. Alternate translation: "it is because they are like a person lost in the dark" (See: [[rc://en/ta/man/translate/figs-metaphor]])