What does it mean when it says a fruit tree must cross pollinate to produce fruit? How is it done?

Question

What does it mean when it says a fruit tree must cross pollinate to produce fruit? How is it done?

Answer 1

The bees do it when they go from flower to flower.

Answer 2

Most pollinating plants have male and/or female parts.  The pollen (male) fertilizes the female parts, but some plants will only produce fruit when the pollen is slightly different (genetically).  This crossing of species is cross-pollination.

Incidentally, some fruit trees will still produce fruit, but not as much, without a cross pollinator.