Dear student...
The aq in brackets means it is the solution in water of the compound..
aq stands for aqueous
for example,
NaOH (aq) means
Sodium Hydroxide (aqueous). It's a solution in water of the compound NaOH.
similarly for HCl (aq)
It's a solution of HCl in water
&
for NaCl (aq)
It's a solution in water of the compound NaCl
Hope helpful to you...