Parliament is the legal body which make laws which effect its country.
The United Kingdom/British Parliament is in London in a building called Westminster. There are to houses which make up our Parliament; Commons and Lords. While some countries within the UK have their own devolved parliament who make their own by-laws the Westminster Parliament has authority and control over England, Scotland, Wales and Northern Ireland.
It is important to remember that Parliament and Government our not the same thing. Parliament makes laws and the government run the country. I.e. Parliament decides on the % level of tax, it is then the governments job to take tax of people, make ads, send letters, employ civil servants to check peoples accounts etc.
When the Tory's (who are the elected government at the minute) do something wrong, then this gets discussed in parliament by all the parties, about where it went wrong, how it can improve and if necessary a change in law is made.
The Prime minster is in charge of the government, while the head of state (Queen) is in charge of parliament, she has the final say on whether a bill (a proposed law) is passed.