Judicial review is a process by which the highest court of the land can examine and ultimately strike down laws made by the legislature, if they are deemed deemed unlawful or in violation of the Constitution.
The process was established following the Marbury v. Madison case (1803), which set the precedent that laws could subsequently be deemed unconstitutional by the Supreme Court.
This on the face of it could lead to the conclusion that judicial review is actually a barrier to democracy, as it prevents democratically elected politicians from passing laws as they see fit, on behalf of their constituents.
However, it is widely accepted that liberal democracies need strong checks and balances in order to limit the power of government. In the case of judicial review, it could be said that the Supreme Court acts as a custodian of the law (and the Constitution) and therefore provides citizens with a further safeguard against a tyrannical parliament.
The idea behind having these checks and balances in place is that rather than stifle or limit the democratic power of the electorate, it actually enhances it. It prevents elected politicians from becoming too powerful or acting with impunity, which should, in theory, lead to a more enfranchised, pluralist society whose rights cannot be easily violated (as they perhaps could be in a country without this legal system).
However, one of the issues which critics have with the relationship between government and the judiciary is its politicization. A great political emphasis is usually placed on the selection of Supreme Court Justices. Even now, the president has stressed the importance of selecting the correct candidates to send to the Supreme Court.
If the justices who perform judicial review are overly political (and too close to the ruling party), the argument could be made that this negates the very purpose of such review—the assumption being that their rulings may fall along partisan lines.