Add and subtract numbers with up to three digits, using formal written methods of columnar addition

What this means, in plain words

A columnar method is the written layout where digits are lined up in their place value columns and worked one column at a time.

A digit is one of the ten symbols, 0 to 9, that all numbers are written with.

Addition puts amounts together to find a total. Subtraction takes one amount away, or finds the difference between two.