RPE is "rate of perceived exertion".
This one is a little tricky to nail down at first because it relies on subjective measure. It's all about you gauging your intensity level (how hard you're working). A 10 would be going as hard and fast as you can go, a 1 would be rolling over in bed. A 2-3 would be a walk in the park. It might be easier to think about it on a scale of 1-5 at first, until you're more familiar with your body's limits.
RPE 1: casual walk, normal breathing.
RPE 2: light sweat, you begin to notice your breathing pace pick up
RPE 3: noticeably sweating, huffing and puffing and you can feel the burn
RPE 4: shirt is soaked, can barely carry on a conversation with the person next to you without them not having a clue what you're saying
RPE 5: max intensity, as hard and as fast as you can go, you're a puddle of water, muscles are jelly and your rest breaks almost require you to sit down you're so spent.
So a 3-4 on a 1-10 RPE scale matches up to about a 2 on my 1-5 scale.
The relative intensity of exercises generally changes from person to person.