Hey there!
verb[short affirmative past tense] + ことがある describes that someone had the experience of doing that verb before.
私はUFOを見たことがある。= I have had the experience of seeing a UFO. (Or more naturally, “I have seen a UFO before.”)
By contrast, 「私はUFOを見た。」is saying “I saw a UFO” which carries a different feel from the example sentence. It indicates one specific time that you saw it.
Japanese Ammo with Misa gives this great example…
日本に行ったことがある。= I’ve been to Japan before.
日本に行った。= I went to Japan. (indicates you went there at a specific time)
去年日本に行った。= I went to Japan last year.
If you’re using the second edition of Genki 1, you can find this grammar point in Lesson 11 on page 256. It’s also here on Bunpro as well if you’d like to see more examples.