Use this phrase to talk about a large number of something.
"Tens of thousands" could mean anything from 20,000 to 100,000, but it seems like it's on the higher end of that range.
You can also say "tens of millions" like this:
We've managed to cut tens of millions of dollars in costs from this year's budget.
By the way, if you want to talk about a general number that's between 20 and 100, you don't say "tens". Instead, you say "dozens". A "dozen" means 12:
She tried on dozens of different outfits.