Explain Like I'm Five

The very short version is

After the Iranian Hostage Crisis, the US placed an arms embargo on Iran so they legally couldn't sell arms to Iran

Reagan wanted to fund the Contra rebels in Nicaragua, however Congress had prevented him from directly funding them so they had to find some way of doing that off the books.

So since Iran wanted to buy weapons, they sold them some weapons so they could turn around and use that money to fund the Contras and circumvent Congress.

They also tried to justify it because they were hoping Iran would influence Hezbollah to release hostages they had taken in Lebanon (which kind of worked, the hostages were released, but Hezbollah pretty much immediately took more hostages, so not really)

This glosses over a lot of details of course.

Why would the US arm people who hate us?

Wait, which time?

My understanding, Extremely simplified:

• Iran had US hostages.
• "But we need guns".
• US wanted to buy some goodwill
• CIA wanted something untraceable cash
• CIA sold guns to Iran for cash they could then funnel into backing the Contras
• Iran accepted weapons, but took their bloody time releasing any hostages
• The agreement had basically devolved into selling weapons for no apparent advantage (Unless you're in the CIA)
• Media learns about the ordeal
• Oliver North gets thrown under the bus as a scapegoat
• Hostages are eventually released
• fast forward a few decades, and Oliver North becomes yet another talking head for Fox News whenever something vaguely familiar to him is in the news cycle (Iran)

You don't know of the great patriot Ollie north?