It's referring to the Douma attack on April 7. Most likely chemical weapons were involved. According to the WHO:
According to reports from Health Cluster partners, during the shelling of Douma on Saturday, an estimated 500 patients presented to health facilities exhibiting signs and symptoms consistent with exposure to toxic chemicals. In particular, there were signs of severe irritation of mucous membranes, respiratory failure and disruption to central nervous systems of those exposed.
More than 70 people sheltering in basements have reportedly died, with 43 of those deaths related to symptoms consistent with exposure to highly toxic chemicals. Two health facilities were also reportedly affected by these attacks.
Furthermore, the BBC reported 20 minutes ago that
The Organisation for the Prohibition of Chemical Weapons (OPCW) inspectors are expected to arrive in the area this weekend.
So we may have further details in the future. Obviously, the US, UK and France held Bashar responsible since they struck at his chemical weapons sites in response, last night, as detailed in the previous BBC link.