It means pinching off a lot of the leaves on a branch (of a deciduous tree) to leave only 4-6 (couple of pairs if they're opposite) so the branch doesn't just keep growing straggly, and so that the new growth at the end of the branch will have smaller leaves (hopefully!) and more of them, as pinching provokes the new growth to split into two new twigs with their own set of leaves - doesn't work for every tree, but for most, or a lot of them. Pinching on a conifer refers usually to growth or buds at the ends of the branches - each type (5, or 2-needle, etc) is done differently - see 'Articles' on
www.evergreengardenworks.com for a great explanation of this.