If you store a list of tags in a row as coma separated values, STRING_SPLIT function enables you to transform list of tags into a table of values. CROSS APPLY enables you to "join" values parsed by STRING_SPLIT function with a parent row.
Imagine that you have a Product table with a column that contains an array of comma separated tags (e.g. promo,sales,new). STRING_SPLIT and CROSS APPLY enable you to join product rows with their tags so you can filter products by tags:
SELECT * FROM Products p CROSS APPLY STRING_SPLIT( p.Tags, ',' ) tags WHERE tags.value = 'promo'
For each row, value of Tags cell will be provided to STRING_SPLIT function that will return tag values. Then you can filter rows by these values.
Note: STRING_SPLIT function is not available before SQL Server 2016