I suggest you take a little time to watch some tutorials. If you are that new to Lightroom that you don't understand what export means then it would probably be a good idea for you to get some other information as well. If you don't I think you'll find Lightroom can be very confusing. You haven't indicated what version of Lightroom you are using. The link I'm providing you is a series of getting started videos for Lightroom 5. And just about everything in this series will apply to other versions. You will note that there is one video devoted exclusively to exporting images. But I think you really need to watch the whole series.
In the LR Library module at the bottom left of the screen there is an Import and Export button. Select all the images you want to convert (Export) a copy to JPG format and click the Export button. In the Export dialog window select the setting you want, JPG as the format, Size/Quality, Whether or not to add the exported images to the catalog, The Location you want the Exported (Converted) images to be placed then click the button at the bottom to start the process.
The information above is correct, but "convert" isn't what is happeneing. Your original photos remain unchanged when you export. When you export, a new file is created with the JPG version of your photo.
By the way, batch exporting of RAW photos to JPG should only be done on edited photos. There's little reason to shoot RAWs and then export un-edited RAWs en masse to JPG.