What is the squiggly mark in senor?

The squiggly mark you're referring to in "señor" is called a tilde (~). It's a diacritical mark that is placed over the letter "n" in Spanish.

The tilde in "señor" indicates a change in pronunciation. Without the tilde, the "n" would be pronounced as a simple "n" sound. However, the tilde tells us that the "n" should be pronounced with a palatalized "ñ" sound like the "ny" in "canyon".

So, "señor" is pronounced as "sen-yor".