what does "|=" mean in c#

Question

I saw this line of code:

RadEditor1.ContentFilters |= (Telerik.Web.UI.EditorFilters)
Enum.Parse(typeof(Telerik.Web.UI.EditorFilters), box.Value);

here

http://demos.telerik.com/aspnet-ajax/editor/examples/builtincontentfilters/defaultcs.aspx

I want to know what "|=" means.

Thank you.

Best Regards,
Kevin Downey