Help Understanding Enumerable.Join Method

Question

Yesterday I posted this question regarding using lambdas inside of a Join() method to check if 2 conditions exist across 2 entities. I received an answer on the question, which

Answer 1

Explanation of the Join.

b = object type of first table o = object type of first table i = object type of second table

1. db.TableB.Where( b => b.MyField == someValue ) This is the element type of the second table
2. o => o.someFieldID The key of the first table
3. i => i.someFieldID The key of the second table (which will match the key in the first table)
4. (o,i) => o The object to return, in this case the object type of the first table.