It depends on why and it depends on where. Being denied tenure for misconduct can be a career killer if it is known, of course. But being denied tenure from a very highly rated place might have little effect except at similar places.
There are some places where it is very difficult to achieve tenure and a much smaller fraction of early career academics make the grade. But those are places where the standards are very high. I once heard a (perhaps apocryphal) story of a place that never tenured anyone, but thought of themselves a a sort of training ground for academics at other institution.
In general, standards for tenure vary and it should be obvious that it will be harder at those places that have a lot of distinguished researchers (and/or teachers). There are even liberal arts colleges in US with such high standards.
So, no, it isn't a block in itself, provided that you meet the standards at the place you are next hired. But you have to produce. There aren't many free rides.
I'll also note that it is possible to be denied tenure for "political" reasons in a dysfunctional department. That has its own problems going forward.
It is even possible to fail to earn tenure for financial reasons. A university suffering a funding crisis, perhaps.
As for how to explain it, you may not need to if your new target is appropriate and the place you didn't earn it was much higher ranked. You will need to explain things like your teaching philosophy and research arc, but that is true anyway.
But, if you don't, then people will make assumptions, which may be fine or not. If your publication and other academic record is appropriate for the new place then there shouldn't be much of an issue.
If you decide to explain it, avoid overly negative words (failed to earn..., denied...) for some more neutral terminology assuming that is appropriate. Perhaps you were told why: "needed more publications", say.