Most apple trees are, to some extent, self-infertile and so heavier crops occur when they are pollinated by other, compatible varieties. Generally, different varieties of apple trees that flower at about the same time will cross-pollinate one another although there are some that both need a pollinator and refuse to repay the favour.
These are “Triploid” apples, so called because the minimum number of apples required (including a triploid) so all bear fruit is three; two pollinators to pollinate one another, either or both of which will also pollinate the triploid.
Confused? You need not be there is a really clear cross pollination chart for apples on the Apple Pollination page on our main site. However, you can make your life simple (and your garden more beautiful by just planting one of the pollinating crab apples. Traditional orchards would include 1 crab apple for every 40-50 fruit trees.
Malus John Downie or Malus Golden Hornet are outstanding for this purpose. These flower freely and will pollinate any apple in an orchard. John Downie has big fruit for a crabapple and Golden Hornet also makes lovely, fragrant crab apple jelly.
Watch your fruit trees grow and enjoy!