Class collaboration is the rejection of the idea of class struggle, instead calling for collaboration and unity between the ruling class and the oppressed classes. While class struggle is a tenet of Communist ideologies, class collaboration is a tenet of Fascism and Liberalism.
Communists aim for a classless and stateless society; they call for workers of the world to unite against their oppressors. Fascism, on the other hand, is more focused on national identity than class identity. For example, German Communists wanted the workers of Germany to unite with workers from other countries to overthrow Capitalism in Germany and other European countries. On the other hand, German fascists wanted the working class and the Capitalist class of Germany to work together, against non-Germans.
Fascism is an ideology which focuses a lot on Nationalism. Communists work towards a world without classes and states. For this reason, fascists see class struggle as a threat. Communists see the Capitalists of their own nation as the enemy, and see the workers of other nations as friends. Fascists see the Capitalists and workers in their own nation as their friends, and Capitalists and workers in other nations as enemies.
As Capitalism strengthens any particular nation to make it powerful within the world, the wealth will mostly go to the Capitalist class. When the workers feel left out of the picture, they will try to overthrow Capitalism. If the workers identify with their class, and see Capitalism as something that oppresses them based on their class, they will fight for Socialism. If the workers identify with their nation, and are convinced to support a Capitalist that wants the nation's prosperity to be more inclusive, they will fight for Fascism.
Class collaboration is not an official tenet of Liberalism, but it works in the interests of Liberalism. Liberalism is the ideology which led to the end of Feudalism and the rise of Capitalism. A class of merchants had emerged in Feudalism, and had overthrown Feudalism and practised Capitalism to serve their own interests. Liberals were in favour of class struggle during Feudalism, but have relied on class collaboration to prevent Capitalism from being overthrown to give way to Socialism and Communism.
Liberal constitutions, especially in any Liberal Democracy, attempt to balance the power and interests of the different classes so that the working class doesn't get too powerful, but also so that it doesn't feel powerless enough to revolt. It allows them to vote and call their political processes "democratic" but does not give them any real power; instead, power rests in the hands of the Capitalist class that controls the economy.
When Communists attempt to overthrow Capitalism, liberals instead demand peace and cooperation. Instead of giving direct power to the workers, they give them welfare to shut them up. They have an obsession with peace and cooperation, and believe that change must be gradual and with the consent of all those involved, including the wealthy Capitalists who exploit the working class.