Now we present the main result, that is (that is to say), the achievable region...
Could you please tell me if it's obigatory to put a comma after "that is" ( and "that is to say") or we can skip it?
The comma is used for setting off nonessential explanations. If an explanation or definition occurs as an appositive it should be set off with commas:
When the nonessential explanation is not at the end of the sentence, like in your example, it should be set off with a pair of commas.
If the nonessential explanation is placed at the end of the sentence then only one comma is needed.