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

You're showing a new employee how to use a computer program. He seems to be confused by it, but you don't think it's that difficult to understand. You want to reassure the new employee, so you say this.

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

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once (something happens), (something else happens)

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.

get to know (something)

This means to gradually start to know more about something (like a computer program). It's also used with people:

Once you get to know Chelsea, she's actually pretty nice.

it's not that (adjective)

This means something similar to "not very". In the example above, the new employee thinks the software is difficult, but you want to say that it's not as difficult as he thinks. So "not that ___" means "not as ___ as you might think".

(something) is hard

This is a more casual way of saying "difficult".