He means that if we don't stand together then we can't fight ourselves and protect but If we do stand together. We will be alright. Because if you divided a house (split) If wouldn't stand up do to unbalanced structure
Explanation: Lincohn means if we all focus on the same thing we will complete something. The House of Representives was divided at this time on the issue of slavery. Using the metaphor of an actual house, the president generally said that if we are pulling in 2 opposite directions something (such as the government) will collapse
