As the names says, this is a "simple" randomization. It means that successive numbers from 1 up to the "number of patients" are randomly assigned to the Letters A, B, C, etc. which here represent the treatments.
In case of simple randomization the letters are used with equal frequency, but there is no restriction for the length of series of identical treatment. It might happen that the last 10 patients are randomized to the same treatment which might be a problem if the drop-out rate is not equal during the study conduction.
With choosing block randomisation this pitfall can be avoid: the treatments are placed more evenly, no long series of identical treatment allocations will follow each other.
Even the allocation ratio can be managed with this randomization method. Defining A, A and B as treatment groups will result a randomization scheme where twice as many A treatment than B treatment will be assigned to the numbers.
Finally, "Seed" is just a technical parameter. Randomization is prepared by so called pseudo-random number generators. The pseudo here means, that the generation procedure is reproducible: identical settings, when even the seed is identical, result the same randomization table.