Two periods (..) mean to concatenate (connect) two things together for the string. So, if UnitName("target") = "b3vad" (string), then "Hello"..UnitName("target") will equal "Hello b3vad". The ! is just an !, since it's in quotes (a string), connected to the rest of the string to print out.
Code:
print("Hello " .. UnitName("target") .. "!")
>> "Hello b3vad!"