Since version 0.15.8 the game displays these probabilities to enough precision (using percentage notation) to see usable values for the uranium processing recipe, but if you're still interested, the exact definition can be found in the game's files.
name = "uranium-235",
probability = 0.007,
amount = 1
name = "uranium-238",
probability = 0.993,
amount = 1
For each batch of ore you process, the result is independently selected according to those probabilities. On average you will get 7 235U and 993 238U out of every 1000, but there is no guarantee of exact numbers.
Since this is much, much larger than the ratio needed for fuel production (1:19), you must do something about the extra 238U.
First, create a generous storage buffer (I recall needing about four steel chests) to keep 238U from clogging things up until you have a better solution.
Research the technology Kovarex enrichment process. This will allow you to convert 238U into 235U, which will not only solve your ratio problem but also allow for immensely more efficient use of uranium ore (and fewer centrifuges).
Note that the actual enrichment process requires that you have at minimum 40 235U as a catalyst which you keep circulating, and preferably more to get the cycle going better despite items sitting idle in belts and input slots. (Players have come up with a wide variety of designs for this.) Therefore, it is wise to save up some 235U early on and not turn it all into fuel, so you don't have to wait again to bootstrap your enrichment.
(Optional:) Research the technology Uranium ammo. This allows you to use extra 238U for another purpose. (The technology Atomic bomb uses 235U so it is not helpful here.)