Spending some two years under siege by al-Qaeda and its allied forces in Idlib Province, Shi'ite civilians from the towns of Foua and Kfraya began a mass evacuation as part of the "town swap" deal, but didn't get far outside of Idlib before they were attacked in a massive car bombing which killed at least 126 people, including 68 children.
The convoy of buses were heading into government territory, but were targeted with the huge explosion in a rebel-controlled area outside of Aleppo. The attack reportedly killed not only a large number of civilians in the convoy, but between 20 and 30 rebels who were present to "guard" the convoy.
The whole population transfer is apparently in limbo after the blast, with rebels accusing the government of "violating" the deal by not sending enough security to escort the rebel fighters and their families out of the towns near Lebanon which are being evacuated on the other side of the deal. As a result, the rebels had stopped the Shi'ite convoy at the time of the attack.
This means that in the wake of the attack, and its huge casualties, thousands of Shi'ites are basically stranded in rebel-held territory not particularly far from the towns they were forced to abandon, and while the Red Crescent is providing food while they wait, many say they've been gone for some 30 hours, and have no access to toilets.