As the other answers have described, lit and typedLit are how to add constant columns to DataFrames. lit is an important Spark function that you will use frequently, but not for adding constant columns to DataFrames.
You'll commonly be using lit to create org.apache.spark.sql.Column objects because that's the column type required by most of the org.apache.spark.sql.functions.
Suppose you have a DataFrame with a some_date DateType column and would like to add a column with the days between December 31, 2020 and some_date.
Here's your DataFrame:
+----------+
| some_date|
+----------+
|2020-09-23|
|2020-01-05|
|2020-04-12|
+----------+
Here's how to calculate the days till the year end:
val diff = datediff(lit(Date.valueOf("2020-12-31")), col("some_date"))
df
.withColumn("days_till_yearend", diff)
.show()
+----------+-----------------+
| some_date|days_till_yearend|
+----------+-----------------+
|2020-09-23| 99|
|2020-01-05| 361|
|2020-04-12| 263|
+----------+-----------------+
You could also use lit to create a year_end column and compute the days_till_yearend like so:
import java.sql.Date
df
.withColumn("yearend", lit(Date.valueOf("2020-12-31")))
.withColumn("days_till_yearend", datediff(col("yearend"), col("some_date")))
.show()
+----------+----------+-----------------+
| some_date| yearend|days_till_yearend|
+----------+----------+-----------------+
|2020-09-23|2020-12-31| 99|
|2020-01-05|2020-12-31| 361|
|2020-04-12|2020-12-31| 263|
+----------+----------+-----------------+
Most of the time, you don't need to use lit to append a constant column to a DataFrame. You just need to use lit to convert a Scala type to a org.apache.spark.sql.Column object because that's what's required by the function.
See the datediff function signature:
As you can see, datediff requires two Column arguments.