mdadm is a tool to create and manage software RAID. It allows you to

create a (new) software raid array. Think of two USB hard drives that you just bought and that you want to use in a RAID 1 configuration.
assemble an (existing) array. Think of two USB hard drives that you have in your shelve and that you have already made to a RAID array using the create command. When you re-attach them to your computer, you need to tell the computer how the disks belong together (RAID 0 or RAID 1 for example). That is what the assemble command is for.

Create

mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sdh /dev/sdi

Monitor

Monitor your raid's status with the command

cat /proc/mdstat