Put in the quotes

Gema, 8 characters

<J>="$0"

Copy of Adám's QuadR solution, just adjusted the syntax and the explanation:

Replace
 <J> one or more lowercase letters
with
 "$0" the quoted match.

Sample run:

bash-5.0$ gema '<J>="$0"' <<< '[abc,def,ghi,jkl]'
["abc","def","ghi","jkl"]

Try it online!

perl -pE, 15 bytes

s/[a-z]+/"$&"/g

QuadR, 10 bytes

[a-z]+
"&"

Try it online!

Replace
 [a-z]+ one or more lowercase letters
with
 "&" the quoted match.