What i don't understand mostly is what is the difference between the stock solution and the concentrate?
I'm assuming you are talking about HC-110.
The concentrate is very concentrated, so you don't need very much of it to develop a single roll of film.
The concentrate is also really thick, so it can be hard to accurately measure and use very small amounts of it.
For that reason, traditionally the instructions recommended that you first dilute a workable quantity of the concentrate into a stock solution (1 +3 probably, but see note below), and then just before you develop the film, dilute that stock solution further (1 + 7) to create your working solution.
You
could dilute the entire bottle of the HC-110 into a single largish bottle of stock solution, but you probably don't want to, because while the concentrate will last a very, very, very long time, the stock solution will only last 1 to 3 months.
So most people either:
1) make a smaller amount of the stock solution up, and then try to use that smaller amount up before it goes bad; or
2) use small syringes or other special tools to each time measure the small amount of concentrate required per roll and then dilute it directly to the working solution.
As an example, say your tank requires 300 ml of working solution to develop a single roll of 135 film. For simplicity, I'll round that up to 320 ml.
If you mix directly from concentrate to working solution, you will need to accurately measure and dispense 10 ml of concentrate and then add 310 ml of water to arrive at 320 ml of HC-110 dilution B working solution.
Alternatively, if you decide to dilute the concentrate in stages (first to a stock solution, and then later to a working solution) you:
a) first dilute a portion of the the concentrate to enough stock to fill a convenient sized bottle. If that bottle is, for example, 500 ml, you would make your (1 + 3) stock solution by putting 125 ml of concentrate in the 500 ml bottle and then adding 375 ml of water to fill it; then
b) when it comes time to develop your film, for each roll of that 135 film, you just need to further dilute enough of that stock solution to 320 ml (in our example) of working solution. That is a 1 + 7 dilution - take 40 ml from the stock solution bottle and dilute it with 280 ml of water, for a total volume of 320 ml.
NOTE: I need to warn you about one further thing. HC-110 comes in two different packagings. Most of us are familiar with the US packaging, and the examples and ratios referred to above and in the Kodak publication I linked to are based on that. There is, however, another European packaging and that HC-110 isn't as concentrated as the US packaging. As far as I am aware, the European packaging isn't labeled in a way that indicates it makes up a quantity of US gallons. You need to determine, however, which packaging you have before determining how much it is to be diluted.