The search keywords are optional, but if this field is populated they will be used for two things:

First, they will be used along with the product name to create meta tags for search engines.
Second, they will be used to assist people when searching your products. Enter keywords separated by commas.

Please refer to Bigcommerce knowledge base for more details: https://support.bigcommerce.com/questions/1062/How+do+I+import+products+and+product+options+with+a+CSV+file%3F