Cross-side network effects are direct network effects that arise from complementary goods or services in a network with more than one side. As opposed to indirect network effects, cross-side network effects refers specifically to the direct increase in value to users on one side of a network by the addition of users to another side.

So to take the example Uber yet again, there large positive cross-side network effects because each additional driver adds direct value to all the users on the passenger side (up to a point) and vice versa.