Well, you can choose not to ever use it in your own code. If it occurs in code written by others though, it's probably more useful to understand it, rather than ignore it.
This syntax is referred to as placeholder syntax for function arguments. When declaring a function literal, you can choose to omit the argument list and the =>. Instead you use _ for each argument. The first use of _ refers to the first argument, the second refers to the second argument, etc.