To add (numbers); to increase a total by adding.
3に2を足すと5になる。
If you add 2 to 3, you get 5.
合計を出すために全員の金額を足してください。
Please add up everyone's amounts to get the total.
To add something; to top up or supplement (e.g., add seasoning, top up fuel).
味が薄いから塩を少し足してみて。
The flavor is weak, so try adding a little salt.
ガソリンが足りないから、途中で給油を足した。
We were low on gas, so we topped up at a station along the way.
Used in phrases like 用を足す to mean taking care of one's business, doing what's necessary; euphemistically can mean going to the toilet.
すみません、少し用を足してきます。
Excuse me, I'll go take care of something for a moment (euphemism for going to the toilet).
買い物に行って必要なものを足してきた。
I went shopping and picked up the things we needed to top up our supplies.