Charlie bars are an extremely simple and cheap way to significantly increase the security of sliding doors and windows. Sliding doors can be insecure because many can be pried open fairly easily. They tend to be secured with small hook locks which can be forced up and out of their loops. A charlie bar will make this technique far less effective.
You can make a charlie bar yourself from an old broomstick or rake handle. Just cut the handle to the right length and place it in the track of the sliding door or window. Heavier stock works better, because a bar that is too light can be bounced out of the track by repeated violent blows.
A more secure variation is a commercial charlie bar which attaches to the door frame. These can’t be bounced out of place. In addition, these can be mounted higher on the frame, which means that you don’t have to bend over to put them in place or remove them. At my age, this is a significant improvement. Commercial charlie bars can be purchased for under $10.