Most people on Strava reserve giving "Kudos" to the rides of others on the basis of achievement. So for instance I recently got 8 "Kudoses" for the Hell of the Ashdown sportive - 100km and 2,000m of climbing (i.e. quite a lot for February). And this seems appropriate.
I give out kudos for people doing similar efforts. Of course this requires you to know what they regularly do so you can spot when they are doing something exceptional. But this isn't how everyone uses it.
It looks like some check in to Strava and simply give kudos to any new activities they can see from people they follow without regard to the effort. So I got kudos for the 20km recovery pootle-around ride that I did the day after the Ashdown effort. Although initially you can regard this kind of kudosing as superficial it's still generally positive or supportive.
Another slightly more disturbing kudosing is when total strangers start giving it to you. This started happening to me when I joined a large, international Strava club. People who weren't following me were giving me kudos - they'd give it once and then never again. How were they finding my activities (and again these could be the simplest little rides)? I finally worked it out - if you go to the club and click on "Recent Activities" you get all sorts of rides that have happened around the same time as yours all over the world - you can then give kudos to people you aren't following.
I know some people find it weird that total strangers are giving them positive input but one way of looking at it is that they are really fellow club members - you are in the same club as them - and they are being "clubby".