Burden means a responsibility or a duty that must be carried out. In law, the word burden is frequently associated with the term "burden of proof." Burden of proof refers to a party's responsibility to provide proof of its claims in a suit through evidence and or testimony. Burden can also refer to anything that restricts or places a load upon. For example, in property law, land can have a burden upon it which requires the property owner to allow a neighbor to travel through the property if it is the way of reaching his house.
A burden is basically a weight, or a responsibility, that accompanies a certain thing. Burdens are often required by law. It means that the particular party who holds the burden doesn't have a choice whether or not they would like to carry the responsibility of the burden. The reason why this is so is because it is oftentimes necessary that certain positions or situations require responsibilities. For example, if there was no burden of proof in lawsuits, then parties would have no obligation to support their claims with evidence. Judges and juries would have essentially no information to make their decisions on.