Using the Quick Search bar in Manage mode, you can use parentheses to refine your searches.
Example:
Canon OR Nikon NOT Rating:1
vs
(Canon OR Nikon) NOT Rating:1
The first search will return items that match "Canon" or "Canon + Rating:1" or "Nikon" (that aren't rated 1). But it will not return items that match "Nikon + Rating:1".
The second search will return items that match "Canon" or "Nikon" but not "Rating:1".
Example:
Canon OR Nikon NOT Rating:1
vs
(Canon OR Nikon) NOT Rating:1
The first search will return items that match "Canon" or "Canon + Rating:1" or "Nikon" (that aren't rated 1). But it will not return items that match "Nikon + Rating:1".
The second search will return items that match "Canon" or "Nikon" but not "Rating:1".