There's a law, as we explained earlier, that a person is supposed to have Kavanah (proper concentration) in the entire Amidah, number one. If they can't, the most important thing is to have Kavanah in the Beracha (blessing) of Avot, the opening Beracha of the Amidah. We have to have Kavanah there; that's imperative.
It's brought down as well that the Beit Yosef quotes this: even if a person did not have Kavanah during Avot, during the first Beracha, there's a second Beracha that also requires great Kavanah and proper concentration, and that is Modim. And that's the halakha (law): if a person is praying, they should, number one, think about having proper concentration during Avot. If for some reason they missed it and didn't have concentration, they should be aware and ready for Modim. Modim is a Beracha that is, according to many, kind of a make-up: if you have concentration in Modim, you're made up for lack of concentration in Avot.
The question is simply, what's so powerful about Modim? There are a couple of points. One, you see that we bow in Modim like we do in Avot; that's an indication of significance. But still, why? Why is it so significant? The Rabbis explain that Avot is the opening of the praise of Hashem, and Modim is the key thanks of Hashem. It's a similar vein, a similar idea, and therefore a similar level of significance.
On top of that, it's brought down that when Mashiach (Messiah) comes, things will change. Mikdash (Temple) will change; different things will change. One thing won't change: our thanks to Hashem and our tefillah (prayer), and our korban todah, the tefillah of thanks and the service of thanks. Both of those will remain. And therefore that shows that Modim, in a certain sense, will be above all when Mashiach comes. That's part of the reason why it's considered a representation of the greatest possible Amidah. If one did not have concentration in Avot as a backup, he can have concentration during Modim and fulfill his tefillah properly. That's the halakha.