Difference between concat and enumerable.concat?

LeI am beginner to the UI path. I have 2 queries What is the Difference between concat and enumerable.concat ? Why we use Variablename.AsEnumerable instead of variablename ?

“Concat” is used for concatenation. String.Concat used to concatenate strings while enumerable.concat is used for concatenation of lists, dictionaries and collections.
The second one definition is selfsame to the prior one.