This means that the second event happens very soon after the first event. The second event naturally follows from the first one. For example:

Once you get to know it, it's not that hard.

We can begin to consider the marketing strategy once we've nailed down the product design.

You don't use "once" when there's a long time between the two events, or if they're unrelated.

After we eat dinner, let's go for a walk.