Some trees and many berry plants are SELF-FERTILE ̶means the insect pollinators or even the wind can pollinate the blossoms without the need of a second tree.
NEEDS A POLLENIZER ̶ means another tree of the same type or kind but a different variety must be blooming nearby at the same time.
EXAMPLE A Liberty apple and a Wealthy apple can cross-pollinate. Two trees of the same variety ie: ̶ 2 Wealthy apples, cannot cross pollinate because they are genetically identical.
Other trees are marked as SEMI-FERTILE. These will set fruit without a second tree. However they will often bear more, and sometimes larger fruit if another variety of the same kind of tree is nearby.
You can select 2 different trees of the same kind marked as NEEDS A POLLENIZER or plant one of those along with one SELF-FERTILE or one SEMI-FERTILE. Also consider ripening times ̶ a Goldrush apple might not start blooming before a Pristine is finished.