Joining strings in lists of strings

You can use StringRiffle:

StringRiffle /@ lis

{"abc def ghi", "jkl mno"}

Use Riffle

lis = {{"abc", "def", "ghi"}, {"jkl", "mno"}};

StringJoin[Riffle[#, " "]] & /@ lis

(* {"abc def ghi", "jkl mno"} *)