What does the term maximum-bottleneck (s,t)-path in the context of maximum flow optimization?
While I suspect that the notion of "bottleneck" has been introduced in an earlier lesson, it's all in the document you link.

Page 8:


  Edmonds and Karp’s first rule is essentially a greedy algorithm:
  
  
    Choose the augmenting path with largest bottleneck value.
  


Page 11:


  For any flow network $G$ and any vertices $u$ and $v$, let bottleneck 
  $G(u,v)$ denote the maximum, over all paths $\pi$ in $G$ from $u$ to $v$, 
  of the minimum-capacity edge along $\pi$.


This sentence is a bit hard to parse. Unfolded:

Let $P_G(u,v)$ be the set of all $u$-$v$-paths in $G$, and $E(\pi)$ the set of edges that comprise a path $\pi$ in $G$. Then,

$\qquad\displaystyle G(u,v) = \max_{\pi \in P_G(u,v)} \quad \min_{e \in E(\pi)} c(e)$.

Note that $\min_{e \in E(\pi)} c(e)$ is the bottleneck (capacity) of $\pi$.

So Edmongs-Karp picks the $s$-$t$-path with the largest bottleneck (in the residual network), which means it picks the path it can send the most flow over (while maintaining conservation of flows).