If you're inexpert at something, you haven't been taught to do it, and you can't do it very well. An inexpert painter might spatter paint all over your wood floors and furniture.
Being inexpert at some activity means you don't have much practice doing it. It takes an inexpert knitter a lot longer to finish a hat than one with experience, and an inexpert driver is still learning the rules of the road. If you're an expert, you're a professional with skill and knowledge, from the Latin root word expertus, "tried, proved, or known by experience." When you add the "not" prefix in-, you get inexpert.