Unless is a word that some people have a hard time with. It means that one statement will be true except for when another statement is true – the statement with ‘unless’ in front of it.
We’ll go out unless it rains.
We will go out – this will happen, except for the case in which it rains.
Unless statements translate basically to ‘if not’, which we can see by translating them to if statements by exchanging if for unless and changing positive statements that come after unlesses to negative ones, and negative ones to positive ones – by reversing the +/-ity of these statements, in other words.
We’ll go out unless it rains = We’ll go out if it doesn’t rain.
A lot of what probably makes it difficult with unless is that it can be used with any mix of positive and negative statements.
We’ll go out unless it rains. (Our original)
We won’t go out unless it doesn’t rain.
We won’t got out unless it rains. (We like rain)
We’ll go out unless it doesn’t rain. (And again)
Note also that as with if it doesn’t make any difference whether the unless part comes first or second in total statement. (At least to the meaning; there might be a difference in emphasis.)
Unless it rains, we’ll go out. = We’ll go out unless it rains.
But in writing, as shown, if the unless part comes first (as with if statements), you need to put a comma after the unless part.