Decades are multiples of 10 (i.e. 10, 20, 30...). Bridging decades involves using multiples of 10 as landing points for adding and subtracting.
For example, 28 + 7 = ? might be solved by bridging 30, with 28 + 2 = 30, then 30 + 5 = 35.
Similarly 35 – 7 = ? might also be solved by bridging 30, with 35 – 5 = 30, then 30 – 2 = 28.
Success in bridging decades is built on a foundation of knowledge about place value structures (e.g. 30 + 5 = 35) as well as number facts for 10 (e.g. 8 + 2 = 10) and numbers less than 10 (e.g. 5 + 2 = 7).
You can read more in the article Mental Methods Moving Along on the AAMT website.
