system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does the letter from 47 U.S. Senators to Iran mean, and why are some saying it's treason?
gpt
The 47 Republican Senators sent a letter to Iran basically saying that any deal they make with Obama (with respect to Iran's nuclear program) will basically be ignored by congress and that those Senators will ensure any such agreement has no effect once Obama finishes his presidential term and leaves office.

Some people claim it's treason because the Senate has no right to interfere with diplomatic negotiations like that. Their job simply involves writing and passing legislation (i.e. laws). Their job does not involve making diplomatic negotiations on behalf of the President / government administration and it's definitely out of line for them to be interfering with diplomatic negotiations like this (especially when they are effectively trying to sabotage those negotiations and roadblock the President's attempts to reach an agreement). 

It's a clear attempt to try and undermine the power of the government administration and the consequences (in terms of damage to diplomatic relations) could potentially be serious. In an international context, it frankly makes the US Senate (specifically the Republican Senators) look immature, ignorant, out of control, politically unstable and disrespectful to international law and foreign policy.