Update August 2017: This offer seems to have disappeared. Roaming is now free in the EU (withing “fair use” limits), as required by law. Rules for roaming in other countries depend on the specific product but even merely receiving calls will incur charges in most places (I checked the US and Turkey systematically and a few others in a more cursory way) and the price to place calls or use data did not strike me as especially low.
Lebara still has some special pre-paid packages to call abroad from your home country (in this case the Netherlands) – which has always been their main market I think – but no special roaming offer as far as I can tell.
Original answer: That seems to be it:
Na uw aanmelding wordt een dagelijkse servicevergoeding van €1 aan u in rekening gebracht voor elke dag dat u Lebara Roaming Services gebruikt.
Rough literal translation:
After your subscrition, a daily service charge of €1 will be charged for each day you use Lebara Roaming Services.
It's not necessarily a bad deal but it's not entirely free and €1 is quite a lot to spend on roaming charges to receive calls. If you are staying in the EU, you must consistently talk for more than 15-20 min a day on average to come out on top compared to other plans (to be completely fair coverage is apparently broader and competing operators might charge more than that outside the EU so if you are frequently travelling in non-EU countries it could still be very attractive). And the duration is also capped at 120 min so it's not unlimited either.
If you use this roaming option for all of one month, that's €30 on top of your regular credit and outgoing calls, which is the price of a premium plan/roaming package in many countries. Outgoing calls are not free and in fact only cheap with Lebara ONE; the charge for outgoing roaming calls with their 10=20 offer is very close to the EU maximum, i.e. not a special deal at all in many cases. And the roaming option apparently does not include data, which is where most people run huge bills today.
Note that this offer only looks too good to be true when you compare it to what many Dutch providers offer but roaming is one area where they are rather bad. And with increasingly tight EU caps, roaming charges are moving constantly and can still be expected to go down IMO.
By contrast, in some places, unlimited calls to other European countries and the US are already included in all but the cheapest plans and in the Middle East there are some reasonably-priced contracts with unlimited roaming in dozens of countries. Last time I looked into it, such offers weren't available at all in the Netherlands, whether for mobile or VOIP landlines.
But it seems perfectly doable, technically and economically, if the experience in other countries is anything to go by so Lebara might simply be trying to import a new model to the Dutch market and I certainly hope/expect even better offers to appear soon, without any catch.