Let me give you an analogy as an example:
Suppose you have to make a list of all the names on the tabs of the files in a filing cabinet. So you open the top drawer of the filing cabinet and start thumbing your way through the tabs, jotting down the name of each one as you go. That's sequential access, and when you do this you can go through the tabs very quickly.
Now let's say you have a list of names and you have to check to see if you have files for them. So you look at the first name, decide which drawer the file should be in, pull open the drawer, thumb through the files to see if the file is there, and put checkmark beside the name on your list if you find it. Now you do the same for the next name on the list, which is probably in a different drawer. And so on. This is random access, and as you can imagine it takes a lot longer to handle each file this way.
"Access time" refers to how long it takes to find a file in the filing cabinet. It includes pulling open the drawer and thumbing through the files to find the one you want.
"Throughput" is how many file tabs you can go through each second when you're just looking at one after another.
In terms of a computer system, "access time" controls how quickly your computer boots up and starts applications when you click on them, because those actions require the computer to find hundreds or thousands of little files that can be located all over the disk.
"Throughput" is most important when you copy large files or use a program to read or write a very large file (note that playing videos or music files doesn't require a disk with fast throughput since the computer doesn't have to read them all that quickly in order to play them at normal speed).
For most people, access time is the measure that is most related to the everyday performance of their computer work. SSDs have access times that are about 100X faster than any mechanical hard drive, and that's why a lot of people are willing to pay a premium for them.