Sempai and Kohai
Let's keep this simple.
"Sempai" is a senior student.
"Kohai" is a junior student.
These are relative terms. We sometimes see "Sempai" being used as a title like "Sensei" is in "Sempai Bob" where Bob is a high grade student probably in training to be an instructor. This is not correct.
Remember that our grading system runs red, orange, yellow, green, blue, purple, brown. If you hold a blue belt you are therefore sempai relative to a yellow belt but you're kohai relative to a brown belt.
Why does this matter? Well it's all about your role within the club. As a sempai you need to set the standards for your juniors and help them out. As a kohai you should respect the journey that your senior friend has taken and look to see what you can gain from them.