The string replacements tab allows you to specify that data coming into a port of the tool service will be used to replace parts of the commands specified in the command tab.
The command is echo Hello %%first_name%% %%second_name%%
This example shows that data coming into the ports first_name and surname will be used to replace parts of the command. For first_name, the Replace port name box is ticked (as by default) and so, any data on first_name will replace %%first_name%% within the commands. For surname, the Replace port name box is not ticked and the string to replace is explicitly specified as %%second_name%%
You can remove a string replacement by clicking the Remove button to the lower right of its area. This will remove both the input port and also stop any replacement of the string.
You can add a new string replacement by clicking the Add string replacement button. You will probably want to change the name of the port.