Clarify the steps: what happened in this mathematical modeling of the TSP?

Source: http://examples.gurobi.com/traveling-salesman-problem

I do not understand this part: (look at the source)

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I understand that $ x_ {ij} $ is equal to 3 but why the “> 2 "?

And what's the problem with subtracting 1 from a set? How do you even do that?

How to come $ | {1,2,3 } | -1 = 3> 2 $ ?!

Good then:
$$ | {1,2,3 } | -1 = 2 $$

So, how is he allowed to write:
$$ | {1,2,3 } | -1 = 3> 2 $$

?

It's basically the same thing as writing: (which is incorrect, right?) $$ 2 = 3> 2 $$

I do not understand this part at all, please explain what happened in language as simple as possible. My level is the final math level of high school.