Time under tension is the amount of time your muscle is placed under the load, basically how long you're lifting for each set. If you count 2 in your head going up, and then 2 going down, that's an average speed, correct?
If you take 4-5 seconds on the eccentric portion (lowering a weight) of a lift, you'll be under tension for a longer duration, which further damages muscle tissue. Don't read too much into what he said, though. It's important to complete the eccentric portion of the lift properly, but no need to worry about it.
Try to do all the positive phases of the lift explosively to teach your body to generate more force, and lower slow and controlled (2-3 seconds).