What is meant by the seal of the prophets :
Seal - To close finally and cannot be recovered.
Therefore the seal of all prophets means the belief in Muhammad as the last prophet
Why is Muhammad so important to Muslims :
Muhammad lived totally as Allah commanded in the Qur'an, so he is the most important person, for Muslims since they see him as a role model for how to live a good life.
He sets the Example of :
1) How to live a Muslim life
2) How to be a faithful servant of Allah