What does the image of the slide below signify?

The notes below taken by your colleague(s) are being made available to you as a courtesy. They are published here on an "as is" basis (meaning I have not reviewed them); therefore, you should use them with discretion to supplement your own notes. If you find these notes useful please be sure to thank the note-takers in person. Remember: gratitude never goes amiss! <-- what does this sentence mean?

Monday-August 27 (al)

Wednesday-August 29 (orz)

Monday-September 3 (school break--Labor Day)

September 5 (orz)

September 10 (orz) September 10-a (orz)

September 12 (at)
September 12 (orz) September 12-a (orz)

September 17 (at)  September 17 (orz) September 17-a (orz)

September 19 -- Test

September 24

September 26 (at)

October 1 (orz) October 1-a (orz) October 1-b (orz)
October 1 (at)  October 1 (al)

October 3 (orz) October 3-a (orz)
October 3 (at)   October 3 (al)

October 8 (at)

October 10--Paper

October 15

October 17

October 22 (at) October 22 (al)

October 24

October 29 (al)

October 31 (al)

Gratitude never
                goes amiss