A Janampatrika records the positions of the planets at the moment of birth. From it, an astrologer reads the lagna, the placement of the grahas and the dasha periods described in the classical texts.
A reading is best treated as guidance within a tradition. It can point to periods the texts consider favourable or difficult, and to the remedies the tradition prescribes. It is not a promise about the future.
Accuracy of the birth time matters. Even a difference of a few minutes can change the lagna, so bring the most accurate record available to you.