Read up on heart-rate zones; this is a decent post[1]. Basically, you should be able to comfortably hold a conversation throughout the run. I learnt that many serious athletes train a lot for it. Improving zone-2 fitness apparently contributes in a non-trivial way to zone 3-5 fitness! But don't get too hung up on these stats (I didn't bother at all for the first 5 months of running), but follow that golden rule of "listen to your body" to differentiate "good pain" vs. "bad pain".

[1] https://www.polar.com/blog/running-heart-rate-zones-basics/

Subtract your age from 180. Run at that heart rate. Another formula 0.7*(HR max — HR at rest) + HR at rest

If you don't believe in formulas, run at a pace in which you can hold a conversation or speak a few sentences without losing your breath.

Zones refer to heart rate zones. You run at different intensities (in different zones) to achieve different things.

Zone 2 is your easy pace. The pace you do most of your running at. It's not so fast where you need much recovery but it's fast enough to achieve the adaptations you need.

Here's part of a talk by famed running coach Jack Daniels on the benefits of easy running: https://youtu.be/veAQ73OJdwY