Welcome to the daily halacha (Jewish law). One of the 39 melachot on Shabbat that you're not allowed to do—one of the acts on Shabbat that you're not allowed to do—is tying, kosher (tying). For example, let's say a person wants to tie his shoes on Shabbat. Halachically, they're not allowed to make a double knot and a bow. If you want to make one knot and a bow, that's fine. But one is not allowed to make a double knot while tying one's shoes on Shabbat. This applies to many other types of clothing that a person double-knots or ties with a very, very strong professional knot. All that is not allowed on Shabbat.
There are exceptions to the rule, says Maran (Our master) Chacham Ovadia Yosef. If somebody usually makes a double knot when they tie their shoes, and then a bow afterwards, they can be lenient if they always untie it within 24 hours. The Chida, who is one of the main poskim (halachic decisors) that we have from the last few hundred years, Maran Chida writes that the Minhag among the Sephardim, the tradition of the Sephardim, is to be lenient when it comes to making double knots on Shabbat. So according to the Chida, even a double knot, a person has a lot of time on Shabbat, their shoes or whatever it is. But anything beyond that, like very professional knots that you have sometimes, let's say a person wants to tie a rope from one wall to another wall, from one pole to another pole, and he has in mind to keep that rope on for a long time as a gate or something, that is not allowed on Shabbat. But if a person just wants to tie their shoes, or, let's say, back in the day, they had different types of belts that they had to tie, made of more of a rope-type material, like a shoelace, all of that—there are a lot of tie, says the Chida, even a double knot—but they should try their best to undo it, to untie it, within 24 hours.
So what's the halacha? Can one make a double knot on Shabbat, yes or no? The answer is no. However, when it comes to tying your shoes, if you will untie them within four hours—for example, if a person ties their shoes on Shabbat morning and makes a double knot with a bow because they're going to take off their shoes in the afternoon or late at night, so they'll untie it—it's less than 24 hours. That knot is only going to last less than 24 hours. Chacham Ovadia says a person can be lenient and tie it on Shabbat. Or let's say it's a child: you can't just make one knot and a bow because it's going to be too loose, it's going to open up, and they can trip. Therefore, if a parent wants to make a double knot on their children's shoes in the morning, it's a halacha as long as they untie it within 24 hours.