If you're a service provider of an Express Service, then you'll be expected to help out your buyers quickly and courteously.
When an Express job is assigned to you, then you must reply to your buyer within 12 hours. If you don't reply within the first 12 hours, then the job will be reassigned to another service provider. You'll also be removed from the service, so you won't be assigned any more Express jobs.
When a job is assigned to you, it is automatically marked as "Started". You'll need to complete the job within 24 hours after it is assigned to you.
To see what tasks you are expected to complete as a part of this service, check out the "what do I get" and "what don't I get" articles for each service here: Express Services FAQs
Keep in mind that your buyer probably won't have a lot of experience with the item that you're working on for them, so you may need to give them some tips to help them get started.