How to Batch Rename Files in a macOS Terminal?

In your specific case you can use the following bash command (bash is the default shell on macOS):

for f in *.png; do echo mv "$f" "${f/_*_/_}"; done

^{Note: If there’s a chance that your filenames start with -, place -- before them[1]:
mv -- "$f" "${f/_*_/_}"}

Note: echo is prepended to mv so as to perform a dry run. Remove it to perform actual renaming.

You can run it from the command line or use it in a script.

• "${f/_*_/_}" is an application of bash parameter expansion: the (first) substring matching pattern _*_ is replaced with literal _, effectively cutting the middle token from the name.
• Note that _*_ is a pattern (a wildcard expression, as also used for globbing), not a regular expression (to learn about patterns, run man bash and search for Pattern Matching).

If you find yourself batch-renaming files frequently, consider installing a specialized tool such as the Perl-based rename utility.
On macOS you can install it using popular package manager Homebrew as follows:

brew install rename

Here’s the equivalent of the command at the top using rename:

rename -n -e 's/_.*_/_/'  *.png

Again, this command performs a dry run; remove -n to perform actual renaming.

• Similar to the bash solution, s/.../.../ performs text substitution, but – unlike in bash – true regular expressions are used.

^{[1] The purpose of special argument --, which is supported by most utilities, is to signal that subsequent arguments should be treated as operands (values), even if they look like options due to starting with -, as Jacob C. notes.}