It isn't clear is it from the import option. It's addressed here https://support.logmeininc.com/lastpass/help/how-do-i-import-from-roboform-lp010041
But it's a bit misleading because you have to go through a CSV either way. My suggestion (based on what I had to do) was as follows:
1) export your Roboform database to a CSV file.
2) open the CSV in Excel (or equivalent spreadsheet program)
3) delete the "MatchURL", "Note", and "Rf_fields" columns, then rename the "Login" field header to "username", "Pwd" to "password", and "Folder" to "grouping" (if you don't have entries grouped in folders you can just delete the column). Column order is not important, just the field header.
Note: step 3 is crucial
because Roboform exports column headers as:
but LastPass expects:
If you just paste in the CSV as exported from Roboform it will only import the Name and URL fields as that's all that will match their import scheme.
Once you hit "upload" LastPass should then recognize either all of your exported data or whatever individual entry you wish to import. LastPass needs to address this in their import routine as it is not explicitly stated.