See here. A soi is what we would call a “lane,” although some have grown so big over time that they actually qualify as a full-fledged street.
I’m familiar with Soi Arab myself, and it is small enough to qualify as a soi. The actual name is the more mundane Sukhumvit Soi 3/1, and it really is quite a crossroads. This area is a center for Muslim tourists. Lots of hotels catering to them and some very good restaurants – Iraqi, Iranian, Pakistani – some even open around the clock. Interestingly, there are also several bars catering to the Muslim trade, allowing them to get their drinking binge out of the way before having to return to their strict homeland. They’re often packed when I pass by.