The "base stat" doesn't really correspond to an specific level. It is actually half of what their level 100 total would be, except for HP which is half of their level 100 total, minus 100.
The number is also without any effect from the nature, effort values, individual values (the hidden values of a pokemon).
So for example, Bulbasaur's base HP is 45 and base Special Attack is 65.
Its lowest HP at level 100 would be about 190 (45x2 + 100).
Its lowest SpAtk at level 100 would be about 130 (65*2).
The numbers are only a guide to show which pokemon are better overall or in particular stats. Their final value for each stat at level 100 can vary by over 100 points!
