In modern Britain the UK Parliament is meant to play the role of checking the Executive or Government and scrutinising what it does.
Political philosophers argued that the separation of powers in a political system was important for democracy. This was so the Executive/Government, the Judiciary and the Legislature are mostly independent of each other and act as a system of checks and balances.
These ideas were fundamental to the American constitution. This was partly as a reaction to the British system. At the time the Monarch was appointing judges. The Monarch also had ministers and election managers in Parliament. They sought, using bribery if necessary, to make sure that the Government won elections.
The two ideas of checking and scrutiny are not the same, though they do overlap.
Parliament will check the Government if it stops it from doing things because it does not agree with them.
Scrutiny is a process by which Parliament examines what the Government is doing and forces the Government to make a reasoned case for its policies and proposals. Parliament should ask:-
If Parliament does this job of scrutiny well then it may have the effect of checking the Government. Why? Because the evidence of things not being done properly, especially if highlighted in the media, may cause the Government to change what it is doing.
The only real means of forcing the Government to change is through a Parliamentary vote and not even all of these are binding on the Government. For example, the vote in October 2014 on recognising Palestine as a state.
Parliament does have ultimate power over the Executive because:-
In practice a Government with a Parliamentary majority can normally get through what it wants.