To give some context for those of you not in Japan: There were 15 political parties in this election. Out of 480 seats in the lower house, LDP won 294 seats. The party that came in second (the DPJ) won 57 seats, and the party that came in third won 54 seats. This huge difference is probably why the expression "landslide" was used.
The LDP does not yet control the upper house. In Japan, legislation generally must pass in both houses. To overrule decisions made by the upper house, 2/3 of the lower house, or 320 votes are needed, which is the reason for the coalition.