Well, as much as I'd suggest to not blindly trust EU websites (been there, done that) you can find all answers you seek on this FAQ on Roam like at home regulation: it's the more recent I can find, and it's straight from the EU itself.
https://ec.europa.eu/digital-single-market/en/faq/frequently-asked-questions-roam-home
Now, link only answers are frowned upon, but at the same time I strongly feel that when in presence of a specific, official FAQ like this we should avoid to fully copy past it here and lose subsequent changes at the original text. Thus, I'm quoting here only a few passages of general interests, valid at least at the day I'm writing, the 2017-06-07 ISO 8601 international date format
Your communications (phone calls, SMS, data) made from another EU
country will be covered in your national bundle: the minutes, SMS and
gigabytes of data that you consume abroad in the EU will be charged or
deducted from the volumes of your national tariff plan
You can roam like at home whenever you are in an EU country other than
the country where you actually live (your effective home). If you move
to and establish a durable residence in another EU country, you will
no longer be able to benefit from roam like at home
The general rule is that as long as you spend more time at home than
abroad, or you use your mobile phone more at home than abroad, you can
roam at domestic prices when travelling wherever in the EU. This is
considered a fair use of roaming services.
If this is not the case, your mobile operator may contact you.
Operators can detect possible abuses based on the balance of roaming
and domestic activity over a fourmonth period
If at home you have unlimited calls and SMS, you will get unlimited
calls and SMS when roaming in the EU.
If at home you have unlimited mobile data or very cheap mobile data,
your operator may apply a safeguard (fair use) limit on data use while
roaming. If this is the case, the operator will have to inform you in
advance about such a limit and have to alert you in case you reach it.
That safeguard limit will be high enough to cover most, if not all, of
your roaming needs. Beyond this threshold, you can continue data
roaming, subject to a small charge (maximum €7.70/GB + VAT; this will
decline gradually to reach €2.50/GB as of 2022).
Again, I strongly advice to check the linked page as there are much more tons of info that those quoted here, which have been quoted just to give a general understanding of the Roam like at home thing.
