In this answer, I will provide one strength for each of the branches of the United States government. Those three branches are the legislative branch (Congress), the executive branch (headed by the president), and the judicial branch (headed by the Supreme Court).
The major strength of Congress is its ability to make laws. No law can be proposed, let alone passed, without the consent of Congress. This means that Congress has the sole authority, for example, to impose taxes and to allocate money for spending. Presidents may have big plans and important proposals, but only Congress has the power to make these plans into actual laws.
A major power of the president (and, therefore, of the executive branch) is what Theodore Roosevelt called the “bully pulpit.” The president is the most famous political figure in the nation. He (or someday she) can command the attention of the public. The public does not really pay much attention when, for example, Paul Ryan gives a speech about proposed policies, but it does pay more attention when the president speaks. This gives the president more power to persuade the people than anyone else in our system.
The major power of the Supreme Court is the power of judicial review. This means that the judicial branch gets to decide what laws are and are not constitutional. For example, the Supreme Court changed the way our political campaigns are financed when it struck down campaign finance laws in the Citizens United case that was decided in 2010. The Court also has done things like making segregated schools illegal (Brown, 1954) and declaring that states cannot ban all abortions (Roe, 1973). The ability to say what the Constitution means gives the judicial branch a great deal of power.