Complex sentence: Kant's theory looks at the act only, and not at its outcomes and consequences, and claims that one is ethically required to consider whether one would be willing to "universalise" the act: to claim "everyone" should behave that way.
Split into simpler sentences: Kant's theory looks at the act only, and not at its outcomes and consequences. You would then have to think whether you would be willing to "universalise" the act: to claim everyone should behave that way.