It is clearly defined in bootstrap documents itself as below:
Body padding required
The fixed navbar will overlay your other content, unless you add padding to the top of the . Try out your own values or use our snippet below. Tip: By default, the navbar is 50px high.
So, give 50 or more pixels of padding to the body to solve the issue. check this fiddle.
Make sure to include this after the core Bootstrap CSS.
body {
padding-top:60px
}