There are subjects with which we are unfamiliar due to a lack of capability, lack of time, or lack of interest. Discussion of these subjects with which we are unfamiliar may leave us feeling "behind the curve".
There are social currents the speed of which is difficult to engage if we operate with intent and analysis rather than the more speedy reflexive banter. Getting "left behind" can also make one feel peripheral.
There are many reasons to feel outside a group such as too tall, short, fat, skinny, old, young, unattractive, poor, or even too smart. One can even be intentionally snubbed or rejected. However, one should be careful not to over value the opinion of others.
There will always be someone smarter or dumber than you. This sort of comparison is rather pointless. It might be more profitable to seek out people with whom you can have an interaction in which you are valued for who you are. Those who are blind to your positive qualities are not worth associating with.