You can think of it as limits as to what you can do. For example, you may right now be able to roll a ball and measure how fast it's moving, using some sort of equipment. You may not think there is a limit to how accurate you can do this -- you may use standard equipment and get a speed of 1mph, then do it again with better equipment and get 1.1mph, then 1.08mph, then 10.086mph, etc. You're just getting more and more accurate, and closer to the true speed.
However, Heisenberg (guy that came up with the maths supporting the idea), said that there was a limit to how accurate you can be, and came up with two equations,
[change in momentum] x [change in position] > planck's constant
[change in energy] x [change in time] > planck's constant
planck's constant is some constant number that appears throughout lots of quantum mechanics. just see it as some number. what the equations mean is that, for example, the more accurate you measure the momentum of something, the less accurate will be your knowledge of its position (at the same time). it's a balancing trick. the same goes for energy and time -- the more you know about its exact energy, the less you know about exactly when in time you measured it.
The reason you never come up to this limit in practical uses (like you measuring the ball, for instance), is that Planck's constant is so damned small. So you can still be 'very accurate' with your measurements, it's just a concept that you have to realise; that it's not an endless journey to being more and more precise.
It's a bit weird but those that use it just get on and use it. It works.