This group was founded by a former member of Al Qaeda. They have Turkish support. This is not an ideal group to be making huge deadway in Syria as while they are not quite as brutal as ISIS (hopefully?) they are still very much jihadi Sunni sharia-types.
Russian troops have abandoned a nearby air base and have been relegated to intermittent airstrikes to try to slow down the advance but thus far it does not seem to be working. Hezbollah was at one point also fighting this group but we all know what's been happening to them over the last few months.
Assad's Syrian Army in the area is now completely cut off from Damascus.
The Syrian situation is so complicated I can't even hope to explain it, with something like a dozen different militia groups and armies fighting for territory. The US sponsored SDF seems to be on the sidelines for this one but they are not friends with this particular group.
One of the Turkish leaders in government said earlier today they are considering flat out invading northern Syria soon, likely to advance against Kurdish groups they claim are terrorists.