The FIFA Disciplinary Committee has sanctioned Barcelona for "breaches relating to the international transfer and registration of players under the age of 18." In consequence, the club has been slapped with a transfer ban at both the national and international levels, lasting for "two complete and consecutive transfer periods."
Barcelona, along with the Real Federación Española de Fútbol (RFEF), were found to have violated multiple provisions related to the international transfer and first registration of non-Spanish minors. The Disciplinary Committee stated that FIFA takes the rules relating to protection of minors very seriously, as those under 18 are "vulnerable to exploitation and abuse in a foreign country."
Because of these violations, Barcelona won't be allowed to bring in new players come summer, or even come January. With the club in the hunt for a new goalkeeper and replacements in defense, such a sanction could certainly have major effects on Barcelona, both in their hunt for another La Liga title and in their quest for a Champions League trophy.
Of course, there's always the opportunity for an appeal, after which Barcelona could very well see the ban overturned. At that point, we'll see just how seriously FIFA takes such violations.