The way I think of them are that they are graphs to chart the story of a flood. the base line or base flow is the normal height of a river. The lag time is the time a river takes to flood from when the time it first rained, this is longer the bigger the rivers channel is as the river can take more water. The rising limb indicates the rivers rising levels, the peak flow is the higest level the river gets too and the falling limb indicates the falling flood levels after the peak flood level.
Hope this helps, when I first learnt it I was a little confused but I got my teacher to explain it to me again and I understood it.