Veil of Time isn't a standard Magic keyword or ability—it doesn't appear in the comprehensive rules or on any official card. If you encountered it, it's likely from a custom/homebrew card, a fan set, or possibly confused with another mechanic.
If you meant something like Phasing (permanents temporarily "phase out," treated as though they don't exist but aren't removed from the battlefield) or Flash (cast at instant speed), I can explain those instead. There's also Time Counters used by Suspend, and effects like Teferi's that interact with phasing and time-themed flavor.
Double-check the exact card name and text. If it's a custom card, share the wording and I'll break down how it works, the rules interactions to watch for, and its Commander applications.