Comment by coloneltaint

They're just functions that you define inside another "outer" function's local scope, and depending on the language they will/can also work as closures - meaning they capture the values of variable in the outer function as well.

I don't know the syntax of kotlin, but lambda functions generally are usually useful to pass as parameters to other, generic/higher-order functions. For example if you have a sort function `sort(listofitems, compareitems_fn)` you could write the `compareitems_fn` as a lambda directly in-line with the call to sort()