Let's start with the line from above that appears simplest:
1 + 2
I'm sure you can tell what this evaluates to just by looking at it, right? 1 + 2 = 3, boring. However, if you take a look at the other lines, you see one common feature that is absent from 1 + 2: the mighty ., or dot. Go ahead and try this:
1.+(2)
Same result. Why is that? In ruby, we use the . to call methods on an object. In Ruby, everything is an object, even things like numbers. So when we write 1.+(2), what we're really saying is "use the + method on the 1 object with 2 as the argument used". This is the same as the following:
1.send("+", 2)
In every case, we're simply sending the message "+" to the object 1 with the argument 2. Thankfully, in order to make math in Ruby more intuitive, smarter Rubyists than I decided to make it easier to write basic arithmetic like we learned in school, so 1 + 2 works just as well as 1.+(2).
# all three of these are equivalent
1 + 2
1.+(2)
1.send("+", 2)