Edit: As pointed out by @Jan_Mantau Strava has changed the weekly snapshot to be a rolling 7 day period instead of a fixed Monday-Sunday period.

They are numbers relative to the (entire) previous week. For each category - # of activities, time, distance - the arrows will point up or down to show if you are above or below the previous week's value, and the number will show you by how much.

One warning: Strava has made the strange choice of counting things across a full 7 day week (Monday - Sunday) rather than a rolling 7 day period. So a lot of the time it will show you all red numbers which are "worse" than the previous week because it is still early in the week. The only day it is truly accurate is on Sunday.

@ActivityFix Are you sure about the last part? In the IOS app they compare with the last week only till the same day of week and even time of day as now. So if its Tuesday 3 a.m. now then the last week will only be considered from Monday until Tuesday 3 a.m.

It may have changed recently, when the feature was first released it was a fixed 7 day period and there were a lot of complaints about it. If it's different now, I stand corrected.

https://communityhub.strava.com/t5/strava-features-chat/feedback-on-your-weekly-snapshot-feature/m-p/23876