Learn Hardcore Polish: Kontakt z klientem bywa trudny. - Contact with the client can be difficult.

Polish does not have articles. So kontakt z klientem can mean contact with a client, contact with the client, or more generally customer contact, depending on context.

What exactly does bywa mean here?

Bywa means something like can be, is sometimes, or tends to be at times. It suggests that this is not always true, but it happens from time to time.

So:

Kontakt z klientem jest trudny = Contact with the client is difficult
Kontakt z klientem bywa trudny = Contact with the client can be difficult / is sometimes difficult

This makes bywa softer and less absolute than jest.

Why is it z klientem and not z klient?

Because the preposition z in the sense of with requires the instrumental case in Polish.

The noun klient changes like this:

nominative: klient
instrumental: klientem

So z klientem means with a client / with the client.

Why does trudny end in -y?