A sensei is a teacher or mentor in Japanese martial arts or other arts, like tea ceremonies or calligraphy.
Senseis are usually respected and wise people.
Examples:
My sensei teaches me karate.
The sensei shows us how to do the tea ceremony.
I listen to my sensei very carefully.
The sensei helps us learn new things.
The sensei has been teaching for many, many years.
I want to be a sensei when I grow up.