FWIW i also know zero Go but the first example made sense (though while i don't know Go, i do know it has multiple return values): the "m[key]" part returns two values which are assigned to the "_, ok" expression which represents a value to be ignored and an "ok" variable which is later used in the "; ok" part (i guess ; separates multiple subexpressions similar to C's "," and the overall expression's value is the last subexpression) and so the two values returned from "m[key]" are the map's value and a boolean (or something that can be used like that) that checks if the value was found. In which case "if _,ok := m[key] {" would be the same as something like "if m.contains(key) {".

Of course this is all guesswork based on similar stuff i've seen elsewhere, so i might be off, but i think despite not knowing the language it still looks readable if you know any other mainstream language (and exercise some guesswork/imagination :-P).