sed is the Stream EDitor. It can do an entire heap of truly cool things, yet the most well-known is text substitution.
The s,%,$,g some portion of the order line is the sed order to execute. The s represents substitute, the , characters are delimiters (different characters can be utilized;/, : and @ are well known). The % is the example to coordinate (here a strict percent sign) and the $ is the second example to coordinate (here an exacting dollar sign). The g toward the end means to all around the world supplant on each line (any other way it would just refresh the principal match).