In the old days 'regrouping' was known as carrying. It's the same idea now as then: moving the tens (or hundreds or thousands) over to the next column.
Addition within 1000 is a free educational video by Khan Academy.It helps students in grades 4 practice the following standards 3.NBT.A.2.
1. 3.NBT.A.2 : Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction. (A range of algorithms may be used.).