What does the "$bigwedge$" symbol mean in "$bigwedge_{j=1,ldots,M,jneq i}Delta_i(x)>Delta_j(x)"$?

It is the conjunction of all predicates evaluated where the index is in the indicated domain.

EG: $lower{1ex}{left[raise{1ex}{bigwedgelimits_{jin{1,2,3}} P_j}right]} ~=~ P_1wedge P_2wedge P_3$

So we are taking the measure for the probability of the event where $Delta_i(chi)$ is greater than $Delta_j(chi)$ for all $j$ in the integer domain ${1..M}$ except when $j=i$.

$$Prleft{raise{1.25ex}{mathop{bigwedgeqquadquad}_{jin{1..M}: jneq i}hspace{-6ex}bigl[Delta_i(chi)>Delta_j(chi)bigr]}right} $$

It is the conjunction, like $P wedge Q$, of all the things. Here $j$ ranges over $1$ to $M$ except for $i$. This will be true when $Delta_i(x)$ is greater than all the rest.