Hi Nelly,
To set this up, add the field for your options to your form and the field you want the "other" description to populate. Once you have entered the values for the checkboxes, select the "other" description field and click "visability rules". There, you can enter logic to have that field appear if the "Other" checkbox value is selected.
You can learn more about how to do this here: https://community.marketo.com/MarketoArticle?id=kA050000000LHX8CAO