As fukawi1 said, reading the respective manual will give you a better idea about how any command works. Anyways, the command "ls" and the switches in the command denote the following (from man page).
As you may be aware, the command "ls" is used for listing the information about files.
l = use a long listing format that will list one entry per line. It will also show the permissions.
a = display hidden files and folders (hidden files in linux start with a . (period)).
s = display the size of each file in blocks.
t = sort the files according to their modification time.
I short, the command "ls -lsat" will give you a vertical list all the files and folders, including hidden ones, arranged according to their modification times.
For the record, the most commonly used switches are -a and -l.