ADDCOLUMNS is a POWER BI Table Manipulation Functions in DAX which is used to add a calculated columns in table.
Basically, ADDCOUMNS returns a table with new column specified in DAX expression.
SYNTAX
ADDCOLUMNS(<table_name>, <column_name>, <expression>
[, <column_name>, <expression>]…)
table_name is a table or any DAX express that returns a table.
column_name is name that is specified to the column, must enclosed in double quotes.
expression is a DAX expression that returns a scalar value, It is evaluated for each rows of table.
Lets look at an example of ADDCOULMNS.
Using ADDCOULMNS to add a new column in table
Here we have a table salesData which is created using a DATATABLE function DAX as shown below.
SalesData = DATATABLE( "SalesId", INTEGER, "Year", STRING, "Amount_Per_Qty",CURRENCY, "Quantity", INTEGER, { {1, 2017, 5000, 4600}, {2, 2018, 5500, 4590}, {3, 2018, 6000, 2345}, {4, 2020, 5600, 1000} } )

Now we will add a new column named “Total Sales Amount” into this table using a ADDCOLUMNS Function. Value for “Total Sales Amount” will be a multiplication of Amount_Per_Qty with Quantity.
Lets add a new columns into table SalesData.
SalesData = ADDCOLUMNS( DATATABLE( "SalesId", INTEGER, "Year", STRING, "Amount_Per_Qty",CURRENCY, "Quantity", INTEGER, { {1, 2017, 5000, 4600}, {2, 2018, 5500, 4590}, {3, 2018, 6000, 2345}, {4, 2020, 5600, 1000} } ), "TOTAL SALES AMOUNT", [Amount_Per_Qty] * [Quantity] )

As you can see, the new column “TOTAL SALES AMOUNT” is added to the table SalesData.
Using ADDCOLUMNS with SUMMARIZE table
You can also use ADDCOLUMNS to add new column in Summarize table as shown below.
Measure_Summarize = SUMMARIZE('Global-Superstore', 'Global-Superstore'[City], "SALES", SUM('Global-Superstore'[Sales]), "Quantity", SUM('Global-Superstore'[Quantity]), "Profit", SUM('Global-Superstore'[Profit]) )
Lets add new column named “Total Sales”, and value for that column will be a Quantity * SALES.
Following DAX expression uses ADDCOLUMNS to add a column into Summarize table and returns new table.
Measure_Summarize = ADDCOLUMNS( SUMMARIZE('Global-Superstore', 'Global-Superstore'[City], Â "SALES", SUM('Global-Superstore'[Sales]), Â "Quantity", SUM('Global-Superstore'[Quantity]), "Profit", SUM('Global-Superstore'[Profit]) ), Â "Total Sales", [SALES] * [Quantity] Â )
Table Manipulation Functions