On daf (=page) 36b, the first Mishnah in this perek (=chapter) taught a basic difference between the sin-offerings that were brought on the outer altar and those brought on the inner, golden altar. Regarding those that were brought on the outer altar, like the sin-offerings of a nasi – a king – or an ordinary person (see Vayikra 4:22-35), even if the actual requirement was to sprinkle the blood of the sacrifice two times or four times, if one sprinkling was done, it would suffice. On the other hand, those sin-offerings that were brought on the inner altar, like the sin-offering of the High Priest or the sin-offering brought by the great Sanhedrin on behalf of the entire congregation (see Vayikra 4:3-21), all of the sprinklings needed to be done properly, or else the sacrifice was invalid.
The Gemara on today’s daf searches for a source for the requirement that sin-offerings brought on the inner altar must have all the blood sprinklings done correctly, and points to the passage (Vayikra 4:20) where the Torah concludes the commandments regarding the sacrificial service of the sin-offering of the great Sanhedrin with the statement that in all ways this sacrifice should be brought in the same fashion as the previously taught sin-offering – that of the kohen gadol. This appears to be a superfluous statement, since all of the requisite laws of the second sacrifice are restated as they were in the passage that taught about the first sacrifice. Therefore the Sages understood the pasuk (=verse) to mean that that all of the laws that were taught need to be carried out precisely in order for the sin-offering to be accepted.
On a practical level, the Mishnah points out one of the ramifications of this law. In the case of a sin-offering brought on the outer altar, since the sprinkling of the blood is sufficient after a single sprinkling, if the owner had an inappropriate thought after that point it does not affect the validity of the korban. Regarding the sin-offerings that are brought on the inner altar, however, inappropriate thoughts would affect the validity of the sacrifice until such time as the entire sacrificial service was completed.