When you search in Gmail the results are returned in pages of 20. If you press Select: All, then you only get the visible 20 selected. What if you want to perform an action on all of the matching messages?
The same thing applies if you want to perform some action on all messages in a Label. You can only see 100 messages at a time, so how do you select the entire Label?
Basically, when you press Select: All, if the label/search results contain more than 100 message, an extra line appears immediately below it, saying Select all [number] conversations in [current view] Click on this to select all messages in the label/results.
Then you just perform the action as normal (delete/assign label etc.)
It’s probably a little risky to preform these actions on search results, since it doesn’t tell you how many messages are selected. Better to create a temporary label and perform the action on the label.