So this is a super late reply, but I was looking for resources on Cassandra for analytics.
When you build an OLAP table, you aggregate over a particular dimension's values, in their example its the time. So in your table you might have
Hour Somethings Other things
0 6 34
1 8 44
2 5 27
3 5 52
... ... ...
24 6 42
The last entry, Θ, represents the sum of all of the above, so you can read it like the total of the columns, so that table might be
Hour Somethings Other things
0 6 34
1 8 44
2 5 27
3 5 52
... ... ...
24 6 42
Θ 144 960
(144 is the total number of somethings, and 960 of other things). I made up all these numbers btw, they don't appear in the video.