If you regularly import a particular type of file (e.g. a payment file from a 3rd party) you can save your fields mappings and reuse them when you next import a file of that type - this is done as follows:
To create the field mappings to re-use when you click on Check Field Mappings setup the mappings as normal then at the bottom of the list of field mappings set 'Save field mapping settings' to Yes and give your mappings a name - as shown below
You can also save the name of the field in the file which contains an external reference or key allocated by the 3rd party - this can be used to match data the next time you import a file of this type - see this article for more information
To use previously saved mappings, when you click on Check Field Mappings you have the option use previously saved field mappings - as shown below - select the saved mappings you want use - your mappings will then be set up - press Save Changes to continue.