I would like to know why Jewish people place stones on tope of thier loved ones grave stone?

Stones were traditionally placed on graves as a mark of respect- it showed that people had been there. It also served an important purpose: Coming into contact with a dead body, or a piece of a body larger than a figure bone renders a person (tamei) impure- and, if a kohen, from being able to offer sacrifices or perform any other duties of a kohen. It would also prevent someone in such state from participating in sacrifices- such as sin opfferings, the Pesach sacrifice, thanksgiving offerings, going up to the temple to during the shalosh regalim (three pilgramage festivals) etc. Thus knowing where graves are is important, more so in the past than now where graves were not necessarily in segregated graveyards, and had to be clearly marked to make sure they were not mistakenly plowed under, making it impossible for a kohen, or somebody on their way to perform holy activities, to cross the field. So the stones left behind by visitors formed a cairn that clearly marked the grave.

Today, it is a mark of respect, a permanent token that the person is remembered and not forgotten.

As an indication that someone has visited the grave.