A bicameral legislature is one that is split up into two houses. Many legislatures around the world are split in this way. For example, the Congress of the United States is split up between the House of Representatives and the Senate.
Bicameral legislatures are generally set up as a way of providing checks and balances in a governmental system. In a bicameral system like that of the US, proposed bills must pass both houses of the legislature before they can become law. This makes it harder to pass legislation than it would be in a unicameral (only one house of the legislature) system.