The Speaker of the House is basically in charge of proceedings in the House of Representatives (the "lower" house of the legislature). No business can get done in the House until one is elected by the representatives. This is the first time in history that a sitting Speaker has been removed from the position in the middle of the term. This is a particularly awkward time since the government will run out of funding in 45 days if Congress does not pass a budget.
This is a result of a growing split between the ultra-far-right and the slightly-less-far-right factions within the Republican party.