Volume
In simple terms, volume describes how much training work you perform.
For resistance training, people commonly discuss the number of hard sets performed for a muscle or movement.
Intensity
Intensity can mean different things.
In strength training it can refer to load relative to maximum strength.
In everyday gym conversations, people also use intensity to mean how hard a set feels.
Important
Heavy does not automatically mean better.
More volume does not automatically mean better.
The right amount depends on your goal, experience, recovery and program.
Beginner Rule
Start with manageable training.
Progress gradually.
Do not add endless sets simply because more feels more serious.