The FIND function finds the first space character (" ") in the name and returns the position of that space in the full name. The number 1 is subtracted from this number to account for the space itself. This number is used by the LEFT function as the total number of characters that should be extracted in the next step below. In the example, the first space is at position 6, minus 1, equals 5 characters to extract.
The LEFT function extracts characters from the full name starting on the left and continuing up to the number of characters determined in the previous step above. In the example, 5 is the total number of characters, so the result is "Susan".
Note: this formula does not account for titles (Ms., Mr., etc) in the full name. If titles exist, they should be removed first.