The dV is the variation of velocity needed to perform a manuever in orbital mechanics (for example manuever node in KSP is measured in how much dV do you need...) Taking for example the system Kerbin-Mun on the picture we can see how much dV we need to perform a successfully landing on Mun. Starting from Kerbin you can see 4500dV, this means that you need about 4500dV to put a spacecraft in LKO (Low Kerbin Orbit), after this we see that on the line which connect Kerbin to Mun there's a "860dV" this dV is the dV that you need to perform a transfer from LKO to a Mun orbit intersection (For example: the velocity when you are in LKO is about 2300-2400 m/s right? Now you have to increase that speed of 860 m/s, so you have to reach about 3160-3260 m/s in LKO to make an intersection with Mun obit. Now you can see another number on the Mun icon, which indicate how much dV do you need to Land and (next to circle) another number which explain how much dV do you need to make a stable orbit around Mun; so for your lander you will need 640(landing dV)+210(orbit dV)+400-700(for human error and to reach escape velocity) To know how much dV has you spacecraft you can use mod like MechJeb or Kerbal Engineer Redux Sorry for bad english, hope you understand