A compound HA when reacting with S, it dissociates as shown below:
HA+S⇌ A− + SH+
What are S and A?
S is Bronsted Lowry base and A is Bronsted Lowry acid.
Since S is accepting a proton, it is Bronsted Lowry base. A is donating a proton, hence, it is Bronsted Lowry acid.