My Iphone 5 shows a plug instead of a lightning bolt when its charging?

My iphone 5 is very new . I just got it a couplke of monhs ago. What I need to know is why, when plugged in, does it show a plug instead of a bolt when charging. What does the plug mean. It cant be the charger because i bought a new one and it still wouldnt work. Sometimes its charging, sometimes it will stop. Can I fix, in any way? For Example if my phone it at 30 percent, instead of a lightning bolt, it will show a plug, then a bolt then the plug again.