What is "safety buffer" or leeway?
Safety buffer refers to the number of days before you will derail on your goal, if you do absolutely nothing else between now and then. This is usually the number of days left in the countdown above your goal. In the image below, the countdown ends in 2 days, 12 hours, and 51 minutes, so we'd say this user has 2 days of safety buffer on it.
When you create a new goal, you get the option to start it with some additional leeway. That basically allows you to decide the date by which you want to have made progress on the goal: if that's today, then you'd enter 0 to make the goal due today; if that's the day after tomorrow, you'd enter 2, to make the goal due in two days.
On these goal types, the safety buffer is sometimes more of an estimate than a statement of fact. It's the number of days until your data crosses the bright red line... assuming that nothing else about your data changes.
On those types of goals, it's best to pay attention to your hard cap limit instead. I've been overachieving here and avoiding fastfood, so I've built up a nice buffer of +3 fastfood meals allowed. The goal thus says I have 42 days of buffer — but if I go to eat fast-food more than three times today, then I can still derail today.
The highlighted number will tell you exactly how many cookies you can eat today, or the highest weight you can report today, before you get on the wrong side of the bright red line. If you stay below this, you'll be safe no matter what, even though your number of safe days may bounce around from day to day.
If you want to plan ahead, the "Hard Cap By Day" table viewable in your goal's Statistics can help:
If you look at the total column, it shows 186 units in total every day -- so that's the cumulative total I'm allowed to reach a week from now. The delta column shows you how much that is: in this case, +3.
Keywords: number of safe days, amount of buffer, how long till you derail