Nitrogen fixers are plants (often cover crops of the legume family) whose roots are colonized by certain bacteria that extract nitrogen from the air and convert or fix it into a form required for their growth. When the bacteria are done with this nitrogen, it becomes available to the plant itself. For best results, inoculate your legume seeds with Rhizobium bacteria. Your local county extension office may be able to tell you where you can purchase Rhizobium bacteria.
When you till the cover crop under, whether by hand or with a garden tiller, the nitrogen becomes available to your lawn or garden or landscape plants. An example of a nitrogen fixer is clover, which is why leaving some clover in the lawn is not a bad idea.