If you roll 1 die, it is totally uncertain which number you'll get between 1 and 6.
If you roll 2 dice and add them together, it is still somewhat uncertain which number you'll get between 2 and 12, but you know you have better odds of getting a 7 than a 2:
If you roll 100 dice and add them all together, you could theoretically get any number between 100 and 600, but your odds of getting 100 exactly are vanishing small (even if all 7 billion people on Earth tried it a trillion times a second, for 20 billion years, the odds of even one of them getting 100 exactly are worse than you simultaneously getting struck by lighting AND getting elected president of the USA).
In fact (being a maths geek, I calculated the above, specifically for this post), you've got a much better than a 95% chance that your total on rolling 100 dice will be somewhere between 300 and 400.
This is what scientists mean by "uncertainty". To a scientist, it doesn't mean "I don't know what the heck I'm talking about". To a scientist, uncertainty is something you can quantify and put bounds on. You can calculate how uncertain you are about something.
You can also add two number together and, if you know how uncertain you are about each number, you can use that to calculate how uncertain you ought to be about the combined total.