I'm not an expert on all the technicalities of League but this is a term I've seen going about frequently these last few months. No idea what it is though.
Can someone explain it to me and why the issue should be concerning?
NotDevan5 votesThis is how I think of it:
Early champion designs were intended to have significant trade-offs. That is to say, champions who got a lot of mobility tended to lack in some other aspect - damage, utility, etc.
Now, mobility is given in addition to having high damage, utility, etc., which makes many (but not all) old, non-mobile champions comparatively bad.
It's not like many of the older champions aren't still playable, and some that are strictly out of place have been or are being reworked, but there is a tendency for newer champion kits to have a lot more options and power than older kits.
Silven1 votesSo basically the term mobility creep describes the early champions that are rather weak because low damage was the price you once had to pay for high mobility.
But that's no longer the case given all the later champions have had both high mobility and high damage output.
Correct?
BastionKross1 votesAlso the frequency of mobility on new champions. As an example:
Back in season 1/2, 
was the League's premier kiting marksman. Her slows shut down the vast majority of champions that wanted to catch up to her. If she played correctly, it was very hard to grab that Ashe for most champions.
In the recent seasons, there's so many jumps, gapclosers, blinks, wall-hops, and other mobility that catching and killing an Ashe is not a difficult task anymore. Today, 
is the League's premier kiting marksman, by sheer virtue of having free, unlimited mobility on every auto attack. Mobility is so common in today's League that in order for a champion to kite they need infinite mobility.
The problem with this is that if you were a champion that had trouble catching Ashe, how are you supposed to ever get near a Kalista? Riot has an answer for you: Play a different champion until we rework the older champions to be more mobile/have more lockdown; which usually takes years.
Silven1 votesThat's sad. :/
Ziovi1 votesmobility creep refer to the tendency of lol's new champions to have good - very good mobility in addition to high damage/utility. On the other hand, old champions often do not have much mobility, and if they have it, they suffer a bit in terms of damage and utility.
Ethereal Sun1 voteshttps://www.youtube.com/watch?v=Bxszx60ZwGw Substitute "Power" with "Mobility" and that's your answear.