HtPi reads as the Pi of Hₜ. ∏ is the product operator — the twin of the summation sign Σ. Where Σ adds, ∏ multiplies; where addition accumulates, multiplication compounds.
The subscript t marks time — H, for Horris, changing across a series. Taken together, ∏Hₜ represents one person's knowledge compounding over time, perhaps gradually becoming a way of thinking in its own right.
In Chinese, it can also be read as "HT派" — the HT school, or school of thought, if you'll forgive the pun. It sounds rather systematic. In practice, it often just means putting a few apparently unrelated things together and seeing what happens.
