As the Cardinals point out there are limited ways to interpret what the pope has said in the encyclical. Either divorce and remarriage now constitutes a valid marriage in which case this would mean the church now accepts divorce as a possibility as opposed to saying that divorce of a valid marriage is never possible. If not this would otherwise mean that two people who are not validly married yet engage in a sexual relationship can still receive communion somehow.
Obviously none of this makes any sense whatsoever and these for Cardinals are rightfully seeking clarification on these very important topics.
The problem is that if divorce and remarriage is considered fine then that would change Millennia of church teaching on the subject. Otherwise it would be saying that sexual relationships outside of marriage are also OK and that a person can receive communion. The only other possibility would be to say that people can receive communion who are in a state of sin. This situation needs to be rectified as soon as possible.
Please see this article for far more detail and information on this subject: http://www.catholicnewsagency.com/news/these-four-cardinals-asked-pope-francis-to-clarify-amoris-laetitia-43614/