Can you please explain to me what means the words, should, would, could exactly?
This is at least two complete lessons. Briefly.

I should go to the store. (I need to, but I might not)

I would go to the store, but I don't have any money to buy anything with. (So, I won't go.)

I could go to the store for more vegetables. (The possibility is there. I may or may not go.)
Exactly? No. There are no "exact definitions". Each of these has multiple usages.

should - it is advisable; it is a good idea (for ... to ...)

You should buy that car. ~ It is advisable for you to buy that car. ~ In my opinion, it is a good idea for you to buy that car.

should - it is to be expected; a person might expect

It should be fairly easy to add these numbers. ~ It is to be expected that it is fairly easy to add these number. ~ A person might expect that it is fairly easy to add these numbers.

could - was able to

When Jack was younger he could lift 150 pounds without any trouble. (He was able to lift ...)

could - would be able to

I'm not sure if I could meet you at 10. How about at 11? (... if I would be able to meet you ...)

would - used to; had the habit of

One thing I remember from living in the Midwest was that the mosquitoes would eat you alive after sunset. (They used to eat ...; They had the habit of eating...)

would - past of will in backshifts.

Lucy will help us with the paper work.
Lucy said she would help us with the paper work.
I knew she would help us. I was sure she would help us.

would - no "definition". would is used to show the imagined result of an imagined situation.

-- How about if we had pizza tonight?
-- That would be great. (Imagine having pizza. Result: Great.)

-- I should probably complain about this.
-- No. I think that would only make things worse. (Imagine the complaining. Result: Things get worse.)

Read up on conditionals for further uses of would.

These are the basics.