It means that they have exchanged services without any payment being involved.
For example, perhaps a computer company provides another company with a computer in exchange for the other company advertising the computer company in their literature.
A photographer who wants to take good photographs of a model but doesn't have enough money to hire one, might advertise for a model and tell the model that instead of money, the photographer will give the model the photographs to use in her portfolio. Both parties involved benefit from the arrangement but no money changes hands.
Student or Learner