What exactly is the meaning of roaming?

It comes from the English: "to roam" means "to wander around". So if you cross the German border and end up in a different telephone network, your telephone costs will be higher.

Do you have to book or reserve anything?

No, you don't have to do anything. Everything runs automatically. You simply pay the same fee as always. If you have a flat rate, you use it just like at home. However, if you are traveling to a country that is not in the EU or one of the three additional countries, you will still have to expect additional costs.

Can I use the data volume just like at home?

Yes, like making phone calls, this is also possible at the same price as in Germany. So you can lie on the beach in Spain and use messenger services such as WhatsApp and Twitter.

Will I receive a message when I run out of volume?

The telephone providers regularly send a text message when the monthly data volume is almost used up. You then have to pay extra for additional volume if you need or want it. This notification is also sent to the vacation destination.

Will there still be text messages: You are now abroad?

Yes, it will continue to exist. It is a message about which foreign network you are in from now on.

Does the discontinuation also apply to phone calls abroad?

No. If you are in Germany and call abroad from there, you will continue to pay higher charges. Unless you have a special contract for that country. The price varies from provider to provider.

Does the abolition of charges also apply to ships?

No. Unless you are close to the coast or on a river and therefore in the mainland network.

Can cell phone contracts be concluded abroad?

No. The "fair use" clause was introduced to prevent cheaper contracts being taken out abroad but used at home.