Sequence is simply writing the steps (or instructions) in a program in the order they need to happen.
Step 1 - Boil kettle Step 2 - Put teabag in cup Step 3 - Pour boiling water into cup Step 4 - Stir with spoon Step 5 - Remove teabag Step 6 - Add milk Step 7 - Add sugar Step 8 - Stir again Step 9 - Drink & enjoy!
The order these instructions must happen in is important to get right. This order is called a 'sequence' in programming - these instructions must happen in the correct sequence.
It would be no good trying to do step 5 before step 2 for example!