This is 1x1 18650 lithium ion battery Cell Holder or Spacer. We have added several images for you better understaneding. This holder provide high strength to the battery from physical damage. For bigger battery this holders are really important to provide or you can say its mandetory. for 1 battery you need 2 holder . Like for making a 1s battery you need 1x2=2 no of spacers. 
 
Package include -
1x1 spacer single pcs