Do you mean as in security from break-ins?
There are fold-down metal bars that block the door from sliding horizontally, as well as little dowels and other items that limit doors' horizontal movement. Likewise, hardware stores sell pins and other devices that stop doors from being lifted off their tracks.
This takes care of the door being slid or lifted open. Of course, a burglar still might be able to smash the door, but that's less likely.
Lock sets can be installed as well (if not currently present) ...keyed locks just like a regular door. one very common theme is a length of "broom stick" or other such stick. This is dropped in the "track" and prevents the door from sliding. Its low tech but effective, of course if they really wanted to get in smashing the window is an option. For that type of security a metal "bars" type of rig is needed.
Drill a hole in bottom of door and stick a nail thru it..cheap and works
@Patti O'Brien Along the same lines, I have a step on lock--not quite like this but the same idea: http://www.homedepot.com/p/Prime-Line-Patio-White-Step-On-Sliding-Door-Lock-U-9879/100199128
The Nail in hole trick works wonders and is easily removed when ventilation is needed. I drilled my hole about 3 inches from the locked position so that I could keep the door open when I wanted air circulation, though I used a 2 inch bolt for added strength. Another option is a shower tension rod dropped in the tracks. You can adjust the tension rod for an appropriate length, again for air circulation, but the rod will prevent the door from opening any further.
Put a piece of wood dowel or something similar in the door track on the inside. Cheap and easy.