It is very dependent on your state and area.
A decade ago I did it between full time teaching jobs, in our little local district that DS was attending. I knew ALL the teachers and a majority of the students, could pick what subjects/grades and days I wanted, and felt that administration had my back. At that time there were still unions, so I not only made my daily salary, but it paid into the state retirement system as well. I was also covered by the union in case anything happened while I was on the job (think about subbing for a trades class, or even p.e. where a student could get hurt).
I would never do it now because subs are considered independent contractors, with low pay, no benefits, no protections, and the minimum of respect. You are responsible for at least providing for the safety of a classroom full of students, and hopefully teaching them a little, as well. Women who I knew who subbed for decades have quit in the past few years because of all of this.
Every state is different, just know that if it looks like "easy money", it often isn't.
Terri