Lingual braces are a treatment method where the appliance (brackets, wires and elastics) used to rearrange your teeth is mounted on their backside (tongue side).
Due to this positioning, they're sometimes called "inside" braces. In comparison, conventional "outside" braces are placed on the side of your teeth that shows.
While the location of the appliance used is reversed, both conventional and lingual technique share many similarities in terms of the type of hardware and orthodontic principles involved.
The chief advantage of having lingual treatment lies in the fact that your braces are non visible.
If someone carefully looks into your mouth they may catch a glimpse of your appliance (especially if you're wearing them on your lower teeth). But otherwise they're not easily detected.
If you feel that wearing conventional braces would simply be too embarrassing, this is one of the "invisible" orthodontic methods (see box below) that you might consider.