I want to use the SQL activity in the workflow and found that there is an option to use Conditional content in the SQL Code.
Just want to know, how to use the field name(highlighted in black) from the temp table in the workflow?
That's a JS tag. The editor control you have open is the standard JS personalization one, <FIELD> just means left hand side of the == op.
NB You generally want to avoid using SQL activities/granting users SQL right, unless there's a perf issue that requires custom non-sproc code to resolve.