Boolean expressions are evaluated in order of operator precedence, from left to right.
The highest precedence operator is not, followed by and and then or. The following expression:

med eq 10 and not destination in (10.1.3.0/24) or community matches-any ([10..25]:35)

if fully parenthesized to display the order of evaluation would look like this:
(med eq 10 and (not destination in (10.1.3.0/24))) or community matches-any <[10..25]:35)

The inner not applies only to the destination test; the and combines the result of the not expression with the Multi Exit Discriminator (MED) test: and the or combines that result with the community test If the order of operations are rearranged:

not med eq 10 and destination in (10.1.3.0/24) or community matches-any ([10..25]:35)

then the expression, fully parenthesized, would look like the following:

((not med eq 10) and destination in (10.1.3.0/24)) or community matches-any ([10..25]:35)