I don’t know what he means by that. I’ve heard open legs referring to externally rotated femurs, but that’s a bad thing. Maybe you’re chopping your stride short. That could be what he means.
As for how to correct it, chopping one’s stride short is usually because you’re not strong enough to open up and stride out fully. Lunges, RDLs, GMs, and ab work will go a long way to fix it. But it will take time. And I personally prefer isometric lifts (yielding ISOs, not overcoming ISOs).