Why do I have uncertainty in my dimension in Solidworks?

"Why" - This is not 'uncertainty' - it is displaying a tolerance. As far as the software is concerned, your part is exactly 14.40mm. The feature that you are using, however, is not used to communicate to the software how large the part should be (that will be the sketch that defined your body), but rather it's used to communicate to a human how large it's OK for the part to be when they make a physical copy. A full description of the reasons for and usefulness of tolerances is outside the scope of your question, however...

"And how to fix" - There's a few ways to 'fix' this.

If you want to change just this once, then you can adjust the displayed tolerances within the 'Tolerance/Precision' dropdown when the dimension is selected. You may need to press the arrow (circled) to reveal the options.

If you want to change is on all new dimensions that you create, then you need to adjust it in the Document Properties

And, if you want to change it on all new dimensions in all new documents that you create, then you'll need to update your default template to include the changes made above to the Document Properties. This, too, lies outside the scope of your question - google how to save your current document as a tempate.