The Parliament (or the State Assemblies) is the legislature of the nation (or state) - that is, they make laws for the country. Laws are the rules we agree to abide by.
Simple, right? Then, why do we need to people specially for this task?
When the country (or state) was formed, some laws were, indeed, made. However, as time progresses, it is realized that there are certain loopholes in the laws, by which the laws can be exploited. Or it may be realized that there are certain activities that should be illegal.
For example, about 50 years back, child labour was not illegal in India. Now, it is illegal. About 25 years back, it was not illegal to sexually harass a male child. A few years ago, it was realized that no laws make provisions for the protection of male children against sexual harassment. And so, a law called "Protection of Children from Sexual Offences Act, 2012" was passed in 2012.
Care needs to be taken to ensure that there are no loopholes in the laws. For example, child marriage was intended to be made illegal in 2006 by the "Prohibitiion of Child Marriage Act, 2006". However, as of 2015, (as per this source), child marriages are legal if they are "voluntary". Therefore, the wordings used in laws can get a fairly complicated.
So, can the Parliament make "any" laws?<a>
No, the laws have to abide by the constitution. It is possible to make changes (known as ammendmends) in the Constitution as well. These changes, too, are subject to a process with the intention of protecting the basic principles of the Constitution.
In addition to making laws, the Parliament (or the State Assemblies), make the financial budget. It is them who decide how to spend the money collected through taxes.